Wiki vs. Blog - what's the difference?

As I am learning more about wikis and blogs, I am becoming just a little more confused. Is there a clear-cut explanation that helps the creator choose which application is best for his (or in my case, her) needs? From what I am experiencing so far - this is a blog - basically an online journal in which I can rant or rave and anyone or no one can read it, and comment if they so desire. But their comments would not change my original rant, they would just add to it with their own comments. If this were a wiki, however, followers are also contributers who can actually edit my rant or rave - essentially changing my words. Of course I could go in and change them back (boy could that become an exercise in futility?) but then you would have to track changes to actually see the exchange taking place. I guess my immediate understanding is that a blog invites commentary, whereas a wiki invites collaboration.