Dynasty- A league wehere you have one main draft and those are your players from now until you cut or trade them. Each year you only have a rookie draft.
Strategy- Youth takes a pretty high premium here as well as quality back ups like Najeh or Musa. Since you will only be adding rookies and WW pick ups to your team, you need to be able to field a team in the future based largely on your initial draft.
Keeper- Similar to Dyansty but you only keep a set number of players, 3 is pretty standard. Then at the start of next year, you announce your keepers, every one else on last year's roster goes back into the draft and a draft is held to fill out your remaining roster needs. Lots of different variations on the keeper league. Too many to list really.
Strategy- Youth again takes a premium. Slightly less than in a dynasty league but for example, most will take LT2 over Priest in a keeper.
Redraft- Draft the best team available for this year because at the end of the season, they all go back into next year's draft. Here is where most will take Priest over LT2.