Which area was hit?
The bombing has focused on the ISIS stronghold of Raqqa, a city in northern Syria. ISIS has had control of Raqqa for more than a year, imposing its brutal interpretation of Islamic law on the city's residents.
The extremists have made the city, which sits on the banks of the Euphrates River, the de facto capital of their self-declared "Islamic State" that stretches across large areas of Syria and Iraq.
Other locations in Syria were also hit in the strikes, a senior U.S. official said, without providing details.
What was struck?
Most of the places hit were hard targets, like buildings, a senior U.S. official told CNN.
"Usually the first part of any air campaign are strategic targets -- fixed locations, big buildings, things that you don't need a guy on the ground to laser-designate," said retired Lt. Col. Rick Francona, an Air Force veteran intelligence officer and CNN military analyst.
A building in the governor's compound, a post office and a recruitment center were among sites hit, activists reported.
There was no immediate word on casualties.
Why is Raqqa the focus of the initial strikes?
The city is known as a place where ISIS houses training centers, weapons depots and accommodations for fighters. During the Syrian conflict, the group has also seized military bases from the Syrian regime near the city and in the wider Raqqa province.
The targets hit by the airstrikes are intended to hurt ISIS' ability to command and control, resupply and train, a senior U.S. military official told CNN's Jim Sciutto.
ISIS has made Raqqa the flagship for its model of governance, providing food, fuel and security to people struggling to survive after years of civil war. But it also imposed hardline Islamist law there and metes out harsh punishment to those who don't follow orders. Locals started calling the city Tora Bora last year, saying it felt as if the Taliban of Afghanistan had taken over.
Who is taking part in the airstrikes?
All the foreign partners participating in the strikes with the United States are Arab countries, a senior U.S. military official told CNN. Those nations are Bahrain, Saudi Arabia, the United Arab Emirates and Jordan.
Diplomatic sources told CNN that Qatar was also involved in the operation, although it wasn't clear whether it actually carried out airstrikes.
"It's a remarkable diplomatic achievement," said CNN Political Commentator Peter Beinhart. "I don't think it was expected that there would be this much Arab support."
How long will the attacks go on?
For a while.
The first wave of strikes was expected to last into the early hours of Tuesday morning in Syria, CNN's Sciutto reported.
It is aimed at striking a strong initial blow, a senior U.S. official told CNN. The aerial campaign on ISIS targets in Syria is expected to continue beyond Tuesday.
Counterterrorism expert Philip Mudd said Tuesday's attacks were "just the start."
"This is not a definitive blow," said Mudd, who previously worked for the CIA. "When this gets interesting to me as a former practitioner is six months down the road, when a second-tier ISIS commander starts to create some sort of cell to recruit foreigners from Europe or the United States or Canada into Syria, do we still have the will and capability, and the intelligence, to locate that person, or that group of people, and put lead on the target?"
What happens next?
U.S. military officials will be trying to assess the effectiveness of the first night of bombing.
Some analysts have suggested that ISIS had already started dispersing its assets and fighters following Obama's warning of action in Syria earlier this month.
The response of ISIS fighters to the initial strikes may give military officials clues on what to target next.
"The follow-up to some of these strikes is -- what are their actions now?" said retired Lt. Gen. Mark Hertling, a CNN military analyst. "There is intelligence that flows from the initial strike," he said.
The Arab governments involved in the strikes could also face discontent among their own people.
"Parts of the population do not agree with Sunni (Muslims) going against Sunni," Hertling said.
The United States is also pushing for a binding resolution at the U.N. General Assembly this week that aims to prevent and track the travel of foreign fighters to Syria.
How has ISIS reacted?
There was no immediate confirmed comment from the militant group on the strikes.
ISIS' official Bayan Radio tweeted a photo that it claimed showed damage to a communications tower in Raqqa. CNN wasn't immediately able to independently verify the image.
ISIS has previously linked the U.S. campaign of airstrikes in Iraq to its decision to execute three of its Western hostages -- two Americans and one Briton. It also recently called for attacks against the United States and its allies.
How have Syrian opposition groups reacted?
Hadi al Bahra, the President of the Syrian Opposition Coalition, welcomed the strikes.
"Tonight, the international community has joined our fight against ISIS in Syria," he said in a statement. "We have called for airstrikes such as those that commenced tonight with a heavy heart and deep concern, as these strikes begin in our own homeland. We insist that utmost care is taken to avoid civilian casualties."
Did the U.S. consult with the Syrian government beforehand?
A U.S. official says it didn't coordinate nor warn President Bashar al-Assad about the attacks. But the regime claims it was given a heads-up.
"According to the Syrian Foreign Ministry, the American side informed the Syrian representative to the U.N. yesterday that the U.S. will carry out airstrikes targeting the terrorist organization (ISIS) in Raqqa," said a banner on the website of the Syrian government's official news agency, SANA.
Beinhart said al-Assad's regime may end up as "the real winner" from the airstrikes on ISIS, since the moderate U.S.-backed rebels aren't "in a position to take this territory that we are pulverizing from the air."
What are U.S. lawmakers saying?
When the strikes began, Congress had already left town to campaign for the midterm elections, and most of the reaction came from those who had pressed the administration to act sooner. Privately, many of them conceded they were relieved not to have to take a vote on a controversial issue just weeks before voters went to the polls in November.
TAMPA, Fla. - U.S. military forces and partner nations, including the Kingdom of Bahrain, the Hashemite Kingdom of Jordan, the Kingdom of Saudi Arabia, Qatar and the United Arab Emirates, undertook military action against ISIL terrorists in Syria overnight, using a mix of fighter, bomber, remotely piloted aircraft and Tomahawk Land Attack Missiles to conduct 14 strikes against ISIL targets.
The strikes destroyed or damaged multiple ISIL targets in the vicinity of Ar Raqqah, Dayr az Zawr, Al Hasakah, and Abu Kamal and included ISIL fighters, training compounds, headquarters and command and control facilities, storage facilities, a finance center, supply trucks and armed vehicles.
To conduct these strikes, the United States employed 47 TLAMs launched from USS Arleigh Burke and USS Philippine Sea operating from international waters in the Red Sea and North Arabian Gulf, as well as U.S. Air Force, Navy and Marine Corps fighter, remotely piloted and bomber aircraft deployed to the U.S. Central Command area of operations. In addition, the Kingdom of Bahrain, the Hashemite Kingdom of Jordan, the Kingdom of Saudi Arabia, Qatar and the United Arab Emirates also participated in or supported the airstrikes against ISIL targets. All aircraft safely exited the strike areas.
Also, in Iraq yesterday, U.S. military forces continued to attack ISIL terrorists, using attack aircraft to conduct four airstrikes. The airstrikes destroyed two ISIL Humvees, an ISIL armed vehicle and an ISIL fighting position southwest of Kirkuk. All aircraft exited the strike areas safely. To date, U.S. Central Command has conducted a total of 194 airstrikes across Iraq against ISIL.
The United States conducted these strikes as part of the President's comprehensive strategy to degrade and ultimately defeat ISIL. Going forward, the U.S. military will continue to conduct targeted airstrikes against ISIL in Syria and Iraq as local forces go on the offensive against this terrorist group.
Separately, the United States has also taken action to disrupt the imminent attack plotting against the United States and Western interests conducted by a network of seasoned al-Qa'ida veterans - sometimes referred to as the Khorasan Group - who have established a safe haven in Syria to develop external attacks, construct and test improvised explosive devices and recruit Westerners to conduct operations. These strikes were undertaken only by U.S. assets.
In total, U.S. Central Command conducted eight strikes against Khorasan Group targets west of Aleppo to include training camps, an explosives and munitions production facility, a communication building and command and control facilities."
Twitter breaks news ... again:
If any doubts lingered over Twitter's capacity to break news first, the overnight raids launched by U.S. forces on Syria on Tuesday night offered another case study of what the microblogging site does best.
The campaign of airstrikes against ISIS in Syria has pitted several Arab countries into action alongside the United States. But there are also some notable absences. Here's the lowdown on who's taking part and why -- and who's staying on the sidelines.