The college teams will start a roster each week of:

1 QB
4 RB/WR/TE (minimum of 1 RB and 1 WR)
1 K
1 Def.

The only restriction is that you must have at least one player on your team from each class (frosh, sophomore, juinor, senior).

The NFL teams will start a roster each week of:

1 QB
4 RB/WR (minimum of 1 RB and 1 WR)
1 TE
1 K
1 Def.

Each team has 1 Injured Reserve slot to use for their NFL players.  If a player shows up on the NFL Injury Report (available in local newspapers and online), an owner may place this player on the IR.  The IR lasts for four weeks however, and if the player is no longer listed on the Injury Report after those four weeks, he must either be dropped or a place must be made for him.  In terms of Draft Value, the injured player retains his draft value until released.  His replacement will not have a DV until the injured player comes off the IR.  The replacement will assume the DV of whoever is dropped at that time.

Trading:
Trading is normal enough.  Both players and draft picks can be traded.  Even college players under scholarship can be traded.  A pro player maintains his DV after the trade.  Therefore, it is possible to have two players on your team with the same DV.  You can keep both players if you're willing and capable of giving up the proper pick and one better.
Example:
You make a trade so that you now have two players with a DV of 6.  Next year, you can protect one of them with a second-round pick (as determined by the DV system) and the other with a first-round pick (one better than the proper pick).  Note that if you have two players with DVs of 5, you can't protect both unless you have two first-round draft picks.