So, it's pretty much a blog, and you can interact with other bloggers by your Tumblr posts. Just like with Twitter when you use #. Or, is that only done by synching your blog posts with Twitter?
Unlike traditional blogs - you have a dashboard where you can see all the latest posts of other Tumblr blogs you follow, so it's like Twitter in that respect. There's also "Reblogs" where other people can reblog your post, similar to a Retweet on Twitter.
I sync my Tumblr posts to Twitter, but it's not required for interactivity.
There's also the tag system. You tag your posts with as many relevant tags as you decide, and on the right hand side of your Tumblr dashboard you can have a list of tags that you follow, so for example, if you choose to keep "Android" on that list then any time you want you can simply click it and see all the posts on the entirety of Tumblr that have been tagged with Android. I have about 25 tags or so, like "Android", "Vegan", "Prince", "Hip-Hop", etc. It's a good and simple way of keeping up with the zeitgeist on certain topics.
Here's one for you
http://www.tumblr.com/tagged/rebecca+black