Football knowledge level recommended for this article:  7 out of 10 Sports fan

The NFL and College game is very similar.  I linked some websites that offer NFL and College gear for cheaper than the league websites if you are interested. The size of the field is the same, the time of the game is the same, and the majority of the rules are the same too.  With the main concepts and rules being the same, here are some key differences to look out for when watching the NFL vs College Football:

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1. The knee down rule: When a player carrying the ball in College has his knee touch the ground the play is over. It does not matter if you were tackled down or tripped when no one was 20 feet from you, if your knee hits the ground the play is over.  In the NFL, however, the play is only over if the player with the ball is forced down, meaning he has to be pushed or tackled or even just touched when he is down.  This can create some unusual and exciting plays in the NFL when a player makes a diving catch he can then get back up and run, for instance.  I believe this rule is in effect in College as to protect players from taking big hits as they are on the ground defenseless.
2. The Catch before out of bounds: A rule that makes College more exciting, in my opinion, is the College one foot catch vs the NFL two foot catch because it allows for more spectacular catches.  This means that a receiver in college only needs to have one foot touch in bounds after he catches the ball before going out of bounds, wile the NFL requires two.  One knee is an acceptable catch in the NFL or College, however.
3. Game Clock: On college the clock stops after a successful first down while in the NFL it does not, unless the player gets out of bounds.  This rule makes too much sense to me because it benefits your team for getting a first down by saving you time when you re trying to make a comeback while the NFL doesn’t give you any added benefit. Again with the game clock, the NFL has a two minute warning where the clock stops at two minutes no matter what, as a bonus timeout for the offense at the end of the 2nd and 4th quarters. College has no two minute warning.
4. Two point conversion: The NFL two point conversion is from two yards out while the College is from three yards out.  Why this tiny difference you ask? Dunno.
5. Overtime: I saved this for last because it is tedious to explain.
   • First off, an NFL regular season game can end in a tie.  The overtime is an extra 15 minute period and is immediately over if a team scores a touchdown or a safety, they win.  A field goal allows the other team a chance to get a field goal or better.  If the game remains tied after both teams have had one possession, then the next team to score in any way wins.  If it is still tied after the period then it goes down as a tie.
   • In college, the teams each get a chance to go 25 yards to score.  No matter what the team does (TD, FG, TO) the other team gets a chance. Unless… the defense on the first possession runs back a turnover for a touchdown they win. If the game remains tied after two of these overtimes the teams are forced to go for a two point conversion after they score a touchdown.