The secretive U.S. drone campaign against al Qaeda and its allies takes center stage Thursday as John Brennan, President Barack Obama's pick for CIA director, faces questions from Congress. As the president's chief counterterrorism adviser, he has made the drone campaign one of his biggest legacies.

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.

The United States has 8,000 drones. The U.S. Army has a robust plan for using them more and more in the future.

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.