Documentation for Crowd 2.0.x. Documentation for other versions of Crowd is available too.
CrowdID is an Atlassian product which makes use of the OpenID protocol to allow you to use a single login for a number of websites. To put it another way: CrowdID is an 'OpenID provider'. You can use CrowdID if your administrator has installed it for your organisation.
This means that you can:
Your organisation can use CrowdID to set up an internal OpenID provider. There are also other OpenID providers, where you can get a free OpenID.