In a nutshell: A keeper league will let you carry a few players from your roster over to the next season. A dynasty league rolls the entire roster (or at least most of it) over.
I don't think they are interchangable. Keeper you can keep anywhere from 1 to 8 players... usually in around 3 to 6 players each year. In a Dyansty... you have one draft and then rookies drafts each year. That and the WW are the only way to replace players. Both have there merits.
I agree...and in Dynasty its more of a long term deal, you draft a lot of prospects, so your roster doesnt look as strong as a Keeper league's at the direct moment in all likelyhood