The database requires that each category (and anything for that matter) be identified with a unique number. The ID number is the number created by the database. If you want to change this, you can go into your database (be careful and back it up first - check the http://codex.wordpress.org/ for backup instructions) and manually change the ID numbers for your categories.
Now, it comes with a cost. If you have a bunch of posts, they will now be listed under the original ID numbers and not the new ones, so you will have to go through and change all of those manually.
There was a brilliant add-on called batch-categories that made this work so much faster and better, changing massive categories at once, but I have been trying to get it to work in the newest WordPress 1.5 and I can't get it to work. I miss it terribly. If you don't have many posts, then this is an easy fix.
The ID numbers only matter when you want to have your list in a specific order, relative to the ID and not alphabetically.