A block occurs when a resource has been locked by an process and another process with an incompatible lock type attempt to acquire a lock on the same resource. Here is the BOL article that has a the Lock Compatability Matrix.
This article includes code (near the bottom) that will show blocks using the DMV's which are the recommended way to find locking\blocking issues in 2005 and newer.
Jack Corbett
Consultant - Straight Path Solutions
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