(CNN) -- The secret U.S. drone campaign against al Qaeda and its allies has transformed the nature of modern warfare, becoming a key weapon in the U.S. arsenal against suspected terrorists. Advocates see drones as an effective tool in the fight against extremists. Opponents worry about civilian casualties and loose oversight.
Here are some key facts about the U.S. drone program:
How does the U.S. use drones against al Qaeda?
Drones are Unmanned Aerial Vehicles. They are used for surveillance and targeted killings, allowing the United States to carry out certain missions without risking the lives of military personnel.
There are numerous types. The MQ-1B Predator is used for what the military calls "medium-altitude, long endurance" missions, offering intelligence gathering as well as "munitions capability." The MQ-9 Reaper is used primarily "in a hunter/killer role," and secondarily for intelligence, the military says. It is designed to carry out the "kill chain (find, fix, track, target, execute, and assess) against high value, fleeting, and time sensitive targets."
Drones are remotely controlled and include visual sensors that allow those operating them to focus in on targets. They carry various types of weapons. The MQ-9 can employ four laser-guided Hellfire missiles.
Outgoing CIA Director Leon Panetta has called drones "very effective" in Pakistan. "Very frankly, it's the only game in town in terms of confronting or trying to disrupt the al Qaeda leadership," he said in 2009.
A Justice Department memo, given to select members of Congress last year, says the U.S. government can use lethal force against American citizens overseas who are operational leaders of al Qaeda or its affiliates. The document provides insights into the Obama administration's use of drone strikes.
The mechanics of the drones have evolved over the years. In early 2003, CNN reported that nearly half the U.S. Air Force's fleet of RQ-1 Predators were shot down or crashed, according to Air Force officials and military records.
Until 2004, drones were used primarily for surveillance in Pakistan. But that year, the CIA fired the first missile from a drone at a terrorist target in Waziristan.
Al Qaeda leader Osama bin Laden warned his associates about drone strikes.
The CIA flew the stealthy UAV RQ-170 over bin Laden's compound in Pakistan to monitor it in advance of the raid that killed him, according to robotics warfare expert Peter Singer.
U.S. officials recently signed a deal with Niger to house surveillance drones in that country to keep tabs on Islamic militants in the region.
How many drone strikes has the U.S. carried out?
The New America Foundation estimates, based on news reports, that the U.S. government has carried out 349 "CIA drone strikes" in Pakistan and 61 in Yemen. The foundation is a Washington-based, non-partisan think tank.
The United States does not release figures on the number of strikes. President Obama surprised many people in January 2012 by officially acknowledging that the attacks even exist.
In the midst of a Google + video chat, he said "a lot of these strikes" have been in Pakistan's Federally Administered Tribal Areas, along the border with Afghanistan, where many members of al Qaeda and the Taliban are known to be. "For us to be able to get them in another way would involve probably a lot more intrusive military actions than the one we're already engaging in," the president said.
Who has been killed by drone strikes?
The New America Foundation estimates that in Pakistan, between 1,953 and 3,279 people have been killed since 2004 -- and that between 18% and 23% of them were not militants. The "non-militant casualty rate" was down to about 10% in 2012, the group says.
In Yemen, the group estimates, between 646 and 928 people have been killed in a combination of drone strikes and airstrikes, and that 623 to 860 of those killed were militants.
Only about 2% of those killed have been high-level targets, the group said.
A study by two prestigious U.S. universities argued that the "dominant narrative" that drones are "surgically precise and effective" is false.
The strikes have killed far more people than the United States has acknowledged, traumatized innocent people and largely been ineffective, according to the study by the law schools of Stanford and New York University.
The Bureau of Investigative Journalism, an independent organization, estimates that 363 "CIA drone strikes" in Pakistan have killed between 2,634 and 3,468 people -- including 473 to 893 civilians.
In Yemen, the group estimates, the United States may have carried out more than 100 drone strikes. Together with other U.S. operations, anywhere between 374 and 1,112 people, of whom 72 to 178 were civilians, were killed, the group estimates.
Obama told CNN that a target must meet "very tight and very strict standards," and Brennan said that in "exceedingly rare" cases, civilians have been "accidentally injured, or worse, killed in these strikes."
Do other countries use drones?
As CNN National Security Analyst Peter Bergen puts it, a decade ago the United States "had a virtual monopoly on drones. Not anymore."
More than 70 countries now have some type of drone -- although only a few possess armed drones, according to The New American Foundation.
Iran has claimed to have an armed drone of its own.
China unveiled 25 drone models in 2010, some of which were outfitted to fire missiles.
"Only the United States, United Kingdom and Israel are known to have launched drone strikes against their adversaries, although other members of the International Security Assistance Force in Afghanistan, such as Australia, have 'borrowed' drones from Israel for use in the war there," Bergen wrote in October.
For many years, Israel led the world in developing Unmanned Aerial Vehicle systems (UAS), according to the Congressional Research Service.
A 2011 study by Aerospace America found 680 UAS programs worldwide.
How else are drones used?
Drones are a rapidly growing form of technology, used for numerous purposes outside the military.
Some law enforcement agencies are using them. Days ago, the FBI used surveillance drones to monitor a hostage standoff involving a 5-year-old boy in Alabama.
The National Oceanic and Atmospheric Administration uses drones to study weather systems and ecosystems.
The Federal Aviation Administration has announced progress in helping integrate UAS into the U.S. aerospace system.
And numerous private companies have sprung up in the last few years to make small remote-controlled mini-aircraft moutned with cameras available for sale.
As CNN Money explains, "Journalists and sports photographers use them in lieu of expensive helicopters. Real estate agents employ them for aerial photos and video. Wildlife researchers and search-and-rescue outfits are using them or studying the potential. Even the utility industry is interested in having them hunt for downed power lines after a storm."