President Obama wants more gun sellers to get licensed by the federal government and run background checks on buyers.

But who, exactly, is a professional gun seller "in the business of selling firearms?" Answering that question will be a tough part of Obama's new plan.

Obama announced a series of executive actions Tuesday that attempt to expand background checks on some gun sales. He wants the FBI to hire 230 additional examiners to process background checks and wants Congress to approve funding for 200 new ATF agents to help enforce the laws.

The president's measures are already sparking debate.

The establishment of background checks is one of the only major gun safety policies that has been enacted by Congress in recent decades.

The FBI, which runs the system, says background checks help keep people safe while assuring legal access to guns.

Gun control advocates say checks should go further. The NRA, the powerful gun lobby, opposes expanding them.

How federal gun licensing works

The Bureau of Tobacco, Firearms and Explosives is the agency that licenses gun dealers.

The ATF defines dealers as people who "repetitively buy and sell firearms with the principal motive of making a profit." The ATF says that people who "make occasional sales of firearms from your personal collection" do not need to be licensed.

The feds also focus on whether the seller presents himself or herself as a dealer. For example, if the seller is advertising, has a business card, or accepts credit cards, those would be red flags for the ATF. Restocking inventory is also a sign.

In some states, these unlicensed sellers can make private sales without conducting a background check. Obama's executive order could reduce the number of transactions that can be made without a check.

The ATF says all dealers must be licensed, regardless of whether they're selling guns in cyberspace, at gun shows, or at brick and mortar stores. "The question under federal law is not where firearm transaction are conducted," says the ATF. Rather, the question "is whether the person conducting those transactions is engaged in the business of dealing in firearms."

How background checks are conducted

All federally licensed gun dealers must run checks on every buyer -- whether a purchase is made in a store or at a gun show.

The checks work like this: A buyer presents his or her ID to the seller and fills out ATF Form 4473 with personal information such as age, address, race and criminal history, if any.

The seller then submits the information to the FBI via a toll-free phone line or over the Internet, and the agency checks the applicant's info against databases. The process can take a few minutes.

Required information includes: name, address, place of birth, race and citizenship. A Social Security number is "optional," though it's recommended. The form also asks questions such as:

-- Have you ever been convicted of a felony?
-- Have you ever been convicted of a misdemeanor crime of domestic violence?
-- Are you an unlawful user of, or addicted to, marijuana or any other depressant, stimulant, narcotic drug, or any other controlled substance?
-- Are you a fugitive from justice?
-- Have you ever been committed to a mental institution?

Of course, many guns are bought and sold illegally; others are sold legally, but without any background check, since the system is only used by gun sellers with a federal license. But in the end, reliable data on gun sales is scarce.

Background checks: By the numbers

The FBI processed a record 23,141,970 background checks in 2015. And December, the month when a mass shooting in San Bernardino shootings claimed 14 lives, set the record for the most checks in one month.

The former annual record was set in 2013, when the twin massacres in Aurora, Colorado, and Newtown, Connecticut -- five months apart in the second half of 2012 -- sparked a gun control push in Washington and a surge in gun sales.

The FBI's National Instant Criminal Background Check System, also known as NICS, operates seven days a week, 17 hours a day, out of a facility in Clarksburg, West Virginia. It is open on holidays, except Christmas. Obama wants to expand it to a 24-hour system.

Since the NICS system was started 17 years ago, the FBI has conducted 225,678,492 background checks. The FBI counts 1,273,232 "federal denials" over the years, most due to the individual's criminal history. Other reasons include "adjudicated mental health" (21,360) and "illegal/unlawful alien" (16,672). The FBI says 68 applicants were denied because they had renounced their U.S. citizenship.

A short history of background checks

Gun background checks were first mandated by the Brady Handgun Violence Prevention Act of 1993. The law was named after Jim Brady, the press secretary to President Reagan who was left paralyzed by John Hinckley's attempted assassination of Reagan in 1981. Brady died last year.

The Brady Campaign, a leading gun control lobby, says background checks work but wants Congress to do more and close exceptions to federal checks. Right now, generally only federally licensed dealers at gun shows must do checks, for instance.

Meanwhile, the Brady Campaign and other groups have helped spur gun laws at the state level.

The last major attempt at federal gun control was about three years ago. President Obama and Democratic leaders proposed a plan to expand background checks and restrict some semi-automatic weapons. The legislative effort followed the December 2012 murder of 20 children and six adults by Adam Lanza at Sandy Hook elementary school. The Senate defeated the bill in April 2013.