SQL Server Locking

Microsoft Link :

https://technet.microsoft.com/en-us/library/jj856598(v=sql.110).aspx

Locks held by Current Processes:
SQL Server 2008 R2
https://technet.microsoft.com/en-us/library/ms189497(v=sql.105).aspx

SQL Team Primer on Locks :

http://www.sqlteam.com/article/introduction-to-locking-in-sql-server

Kalen Delany Crisp explanation on Update Locks:

http://sqlblog.com/blogs/kalen_delaney/archive/2009/11/13/update-locks.aspx

UPDATE locks are not really a separate kind of lock, but rather are a hybrid of SHARED and EXCLUSIVE locks. And contrary to what you might think, UPDATE locks are not just acquired for UPDATE operations.  A transaction acquires this kind of lock when SQL Server executes a data modification operation but first needs to perform a search to find the resource to modify.

Locks Acquired by a SPID:
Quick and dirty way but gives you good idea about resources involved.

SQL Server Locking Diagram

SQL Server Locking Diagram