An alternate universe twitter is probably a better comparison. Content largely tends to be images or video but can be text posts as well. It started off much less "federated" where you were running your own "blog" on Tumblr that had it's own styling, but it's become increasingly centralized.

"Reblogs" are a bigger feature on Tumblr than twitter generally and the reply / threading structure is pretty unique, which has a tendency to create a lot of blogs that are essentially 100% reblogging other content.

Pretty much. With blackjack and hookers...