Deadlocking occurs when two user processes have locks on separate objects and each process is trying to acquire a lock on the object that the other process has.

In SQL Server when a Deadlock situation occurs it select the processes which have the least overhead to rollback to abort. This way deadlock is automatically resolved. There are multiple ways to identify deadlock i.e. Profile Deadlock Graph, DMV - sys.dm_tran_locks, Extended Events.