What is the JLB position in Football?

Question

I was reading a magazine on college football and I noticed it said Alabama played with a JLB and I was wondering what was it what are his duties on the field?

Answer 1

JLB = "Jack" Line Backer.  The "Jack" refers to an imaginary box within which this linebacker can roam freely.  This position is used more in the AFL (Arena Football League).

More info (from wikipedia)...

Three defensive players must be in a three- or four-point stance at the start of the snap. Two defenders serve as linebackers called the mac and the jack. The mac may blitz from the side of the line opposite the offensive tight end. The jack's role has changed after new rules set in place by the league in 2008. The jack cannot blitz but under new, more defense-friendly rules, the jack linebacker may roam sideline to sideline within five yards of the line of scrimmage and drop into coverage once the quarterback pump-fakes.  (Before this rule, the jack could not drop back into coverage until the ball is thrown or the quarterback is no longer in the pocket and the jack had to stay within the box designated by the outside shoulders of the offensive line, the line of scrimmage, and 5 yards back from the line of scrimmage.)

Answer 2

I'm glad Hoosier Daddy knew it, I thought it was a typo for ILB or something.