Robots/Spiders visitors ??
 

Googlebot (Google)
LinkWalker
Unknown robot (identified by 'robot')

Whats that ?

that are spiders from search engines. They visit your site and analyse it. This way they determine how you should appear on their search engine.

Some are doing the SE work - like the Googlebot one - some are looking for email addresses, and harvesting links for auto-generating emails to you or others(definitely something to look at if you are getting alot of the bad ones is to ban them using robots.txt) - I'll post a list of the ones I know in a bit