Hey Steve,
The custom title tag will change the page title in the tab or browser bar at the top for that specific page when it is viewed. See up top in your browser where it says meta this, meta that . . . in the tab or bar? - that.
The meta description is a short overview for the content on that page or post. It is the default description for that page that shows up in search results.
The meta tags are basically keywords relevant to that specific post.
The post tag is what is used for your tag cloud. For instance if you wrote a post about me you could use the post tag "clown" and when someone clicks on the word clown in your tag cloud I would magically appear. Or, you know, the post would show up on the page with every other post tagged with clown.