PicketLink IDM aims to provide a common identity model for Identity Management operations. This includes easy management of identities like Users/Groups/Roles and their attributes. Framework also provides usage of different identity stores like LDAP or RDBMS.
The best way to learn about IDM component design is to read first two parts of ReferenceGuide:
For the quick start we suggest downloading a simple maven project example from svn. It contains a number of example API usages and comes with embedded HSQL database and OpenDS LDAP servers so you can start playing with the code immediately!