vBulletin Manual

A spider is a program or script that systematically follows URLs on a page gathering information.

Typically search engines use spiders to gather information from web sites to increase there search index.

example search engine spiders:

Googlebot
lycos
ask jeeves
scooter
fast-webcrawler
slurp@inktomi
turnitinbot

Normally these spiders show on your board as guests. If you want to be able to see search engine spiders that are browsing your forum pages there is a setting under the vBulletin Options section in the AdminCP.

AdminCP > vBulletin Options > Who's Online Options

Enable Spider Display - Setting this to Yes will allow you to see the spiders you have specified in the Spider Identification Description
Spider Identification Strings - Enter the unique identifier for the spider you want to see
Spider Identification Description - the description of the spider to be displayed in the Who's Online section of your board