Weird usernames on my Drupal site
Yes those are most likely bots; you can never be 100% sure but if you're seeing a pattern, and these users aren't contributing to the site in the same way regular users do, it's likely.

I think anyone who's ever administered a medium-large Drupal site has experienced this at one time or another. Standard tools for the arsenal include:


Botcha


  BOTCHA is a highly configurable non-CAPTCHA spam protection framework.

Spambot


  Spambot protects the user registration form from spammers and spambots by verifying registration attempts against the Stop Forum Spam (www.stopforumspam.com) online database. It also adds some useful features to help deal with spam accounts.



Both are very effective in my experience, though Spambot can get fairly intense on outgoing http requests if you get a lot of registration attempts. There are some others listed on the Spam control modules page too.