A 'selector' is mechanism to allow a single domain to have multiple keys. Some domains, like Twitter and eBay, use dkim. Google Apps domains typically use google. Others simply use default as a value.
If you have a message signed with a DKIM key, you can find the selector by looking at what is defined for the s= value in the DKIM-Signature header. In this example, the selector for the domain agari.com is s1024: