What are Dynasty rankings in fantasy football?

A dynasty league is very different than the traditional redraft leagues that most fantasy players participate in. A dynasty league is one where you get to keep most or even all of your players from year to year. Just like the NFL, this gives an owner the opportunity to build up a team over time. Also, just like in the NFL, a bad pick (or conversely - a good pick) can impact the team for many years.

Difference between Dynasty and Keeper Leagues?

While both dynasty and keeper leagues involve keeping players, the difference is in the number of players that can be kept. Keeper leagues generally only allow a handful of players to be kept while the rest of the last year's team roster goes into the pool for drafting. Dynasty leagues allow for more players to be kept.