What qualifies as 'perjury?'

Faulty memories don't constitute perjury.

Deliberately lying under oath does.

Not telling all you know? Well, that's a bit trickier.

If conversations between Roland Burris and Rod Blagojevich's representatives about Burris' interest in filling Barack Obama's U.S. Senate seat merely slipped the mind of Illinois' former attorney general, he may be guilty of nothing more than a memory lapse. But if Burris deliberately made false or misleading statements under oath during questioning by members of the Illinois House impeachment committee last month, he can be charged with perjury.

To be found guilty, prosecutors must prove a person testified falsely while knowing his or her statements to be false.

Perjury charges arise when people try to conceal the truth, says Ronald Allen, a Northwestern University law professor.

"If you're not trying to conceal the truth, it's not perjury," he says.

Omission is a bit more murky.

"The farther you get away from the paradigm cases, intentionally telling a lie knowing full well what you're doing, the more likely you're outside the scope of the statute and the harder it is to get a conviction," says Allen.

The sentence for a perjury conviction can range from probation to prison time.

"Theoretically, it can be a fair amount of time in jail, although that doesn't happen very often," says Allen.

State or federal prosecutors have to bring charges, says Allen, and typically they have other cases they consider more important.