Bots are robots that provide some sort of functionality within a Google Wave. You just drag and drop them just like any other contact onto your wave and you can begin using them. The one weird thing I have found is that there does not seem to be any way to find and add them from within Google Wave. So after a little Googling here are some of the useful ones I have found:
Preview Bug: If you copy and paste the email address into your add contact dialog, Google Wave might not enable the Submit button. If this happens, just delete one of the letters and retype them and it should enable the submit button.
firstname.lastname@example.org - Used to interact with blog-posts in Blogger. This is a very useful bot.
email@example.com - Used to interact with Wikipedia
firstname.lastname@example.org – Used to interact with Twitter. Add new tweets or search for tweets. Very cool
email@example.com – Retrieves information from Amazon.com.