Tumblr is more a community than a blogging engine.
If you're registered on WordPress.com, chances are you write a blog. Whether you're a literature fan, a passionate photographer or a technology enthousiast, you produce content for others to read. And those readers aren't necessarily (and probably aren't) registered to WordPress.com because consuming this content doesn't require to be. You either subscribe to the RSS feed or visit the blog daily. You're a visitor.
Tumblr holds the same features, but adds two new ones: * the Dashboard * the Reblog
The Dashboard is like a homemade RSS reader: you follow Tumblr blogs and find them aggregated on your Tumblr home page. It's a consumer feature. The only difference with a regular RSS reader is that you're limited to Tumblr blogs. But because there is that second feature, users don't feel the lack.
The Reblog is quite particular: you produce content on your blog without producing any. I considerer it a product because although your blog will publish it as a duplicate, any visitor of yours will probably only see this version and will consume it as though you were its creator. So Reblogging is production after consumption: you've seen something on a Tumblr blog, you republish it on your own. It's not a simple link: it's a complete new post.
The only difference with a normal post is the presence of a source link (usually preceded by the word "Via"). This source link is the Tumblr post from which this Reblog comes from, but it's not necessarily the original post. And when, after having clicked half a dozen of "via" links you finally end up on the original post, you still don't know the source. Why? Because posting on Tumblr doesn't require to. This attitude is emphasized by the use of the bookmarklet, which grabs any image of the current page and uploads it anonymously on Tumblr's servers.
So when I find a beautiful picture of which I want to find similar ones, and I finally reach the source, I still don't know where it comes from, because the source according to Tumblr is the original post, not the content's origin.