Since there's varying ranges of experience with American football on the site, I thought I'd start a topic to discuss offensive formation. I'm going to go through the basics, hitting on ones I see used often in Grid-Iron.

Wikipedia article with diagrams of several offensive and defensive formations: http://en.wikipedia.org/wiki/Formation_(American_football)

I Formation
- The wiki article shows the basics of this formation with a diagram of the traditional I. This is the basic formation that the sim sets up for its games. It is one of the most common formations in use today by American football teams. It is a versatile formation and almost every team will use it from time to time.
- This formation can also be adapted to your playing style. Two TEs can be used (having only one WR) for teams that like to run the ball more often, adding an extra blocker on the line of scrimmage. Also, you can exchange the TE in the base formation to have 3 WRs, giving you an extra threat in the passing game at the expense of a blocker. TEs can also catch passes of course, but typically a WR is better at this and faster. You trade the better blocker in the TE for a more potent receiving threat in the WR.
- The FB can be off-set to the right or left in this formation as well, giving him a head start in lead blocking on outside runs or protecting the weak side of the line from blitzers. (Called Weak I or Strong I depending on which way the FB is off-set)
- I see lots of teams that rely on a heavy run offense using two TEs in G-I.

Wishbone
- Utilizing two halfbacks (HBs, also called tail backs) split right and left and a FB in front and between them, and usually one TE and one WR on opposite sides. With the running game being ahead of the passing game at this stage, versions of the wishbone seem to be popular on G-I.
- This is an offense designed to run the ball with 3 backs in the backfield behind your QB. Passing does happen out of this formation, but when it is run, these passes are typically on options (decision made by the QB whether to run, pitch to the HB, or throw to the WR).
- The Wishbone, while not used in the NFL since the parity among athletes doesn't make it a very successful offense there, is still used at the college and high school level. There have been a few variations of the wishbone, most commonly seen is the flexbone that utilizes wingbacks (not possible in G-I). Also, some teams forego the WR, replacing him with a TE for an extra blocker in the rushing game.
- Variations of the wishbone appear to do well in G-I with the proper personnel and planning. If you want to utilize this formation, running the ball will be your strength.

Pro Set (or Split Backs)
- The Pro Set utilizes two running backs split to either side behind the QB. It is essentially the same as the I-Formation, except that instead of having a blocking back (FB), you have two runners. Often teams that run this set utilize their backs as receivers as well.
- This set is very versatile, like the I-formation, and is utilized to take advantage of having two, not one, talented runners.
- This set is very popular in G-I, and general thought is that by using two runners instead of one, fatigue is less of a factor late in games. It also has the benefit of getting experience for two HBs every game instead of just one (important should your #1 HB go down with an injury to have a #2 that is of nearly equal skill).

Ace Formation (or Singleback)
- A formation that utilizes only one running back (HB) lined up behind the QB. The basic Ace formation employs 3 WRs and 1 TE. This formation is typically used by offenses that pass more often, but can be used to run out of as well. Running success in this formation is typically drawn from pulling a linebacker out of the middle of the defense to cover a WR or TE.
- Variations of this formation typically change with the number of WRs and TEs used. Teams that pass often or find themselves in a position where passing is their best option (3rd and long for example), will switch the TE for a 4th WR. This forces the defense to cover 4 speedy receivers rather than the normal 3. Also, some teams use this offense with a second TE, sometimes referred to as an H-Back when lined up behind the line of scrimmage. The Indianapolis Colts use this formation, and the second TE (H-Back) is used as a third option receiver most the time while the other TE blocks.
- Not as common in G-I as the other sets, but it is used by several teams, particularly ones that favor the pass.

There are certainly more formations than this, and certainly several ways they can be implemented. The wikipedia link lays out several old and new offensive and defensive formations in use at various levels of competition in football. These three cover the most common ones I've come across in G-I. Also, there is never a right or wrong set up. I thought it might be helpful to have a topic discussing ways to set up your offense since there may be several players that aren't as familiar with it as others.

Wikipedia is so great