difference between lock, block and deadlock

Lock: DB engine locks the rows/page/table to access the data which is worked upon according to the query.

Block: When one process blocks the resources of another process then blocking happens. Blocking can be identified by using
SELECT * FROM sys.dm_exec_requests where blocked <> 0
SELECT * FROM master..sysprocesses where blocked <> 0

Deadlock: When something happens as follows: Error 1205 is reported by SQL Server for deadlock.