The Dolphins' base defense is as Muck said. Here is a link to an excellent introduction to the "cover 2" defense.
http://espn.go.com/ncf/columns/davie/1437187.html
Because Miami's cornerbacks are among the best in man coverage, they play a lot of press coverage, where the CBs play up on the line of scrimmage to bump the WRs as they're trying to get into their pattern. That often disrupts the timing or the route, giving the defensive line a little more time to get to the QB. If they don't get the jam, a big play is usually the result.
Because Miami's front seven has been tough against the run, teams usually find it easier to score by spreading the field and throwing the ball. If the other team goes with 3 WRs, we'll take out Morlon Greenwood and bring in a 3rd corner. If they go with 4 WRs, Junior Seau comes out and we put in another DB. Last year, our 3rd corner played more than Greenwood did.
One of the more interesting formations the Dolphins used last year was the "Tiger" formation, which was a 3-3-5 formation where Jason Taylor lined up at MLB and could blitz from wherever he wanted. I'm a little hazy on the other details but I seem to recall Seau being used as a DE in that formation.
In general, if an offense wants to throw, they'll put more receivers in the formation. When they do that, you'll want more DBs out there. If an offense wants to run, they'll put in their bigger guys to block (FBs and TEs). When they do that, you'll want more LBs (and even DLs for a goal-line stand).
Passing game = little quick guys on both sides
Running game = big strong guys on both sides
