SP2010 user profile synchronization problems / Forefront Identity Manager (FIM)

Btw, official link : http://technet.microsoft.com/en-us/library/ee721049.aspx (this isn't trivial stuff)

What happens : behind the scenes, SP2010 uses FIM, which is deployed to the hive (\14). There is an integrated UI to debug issues there : \Program Files\Microsoft Office Servers\14.0\Synchronization Service\UIShell\Miisclient.exe

Invaluable to solve issues (the most common being your account not having AD replication rights)