Sql server - Describe in brief SQL Server locking

Describe in brief SQL Server locking.

SQL Server has 3 main lock types:
- Shared: Locks are compatible with other shared and update locks.
- Update: Locks are compatible with shared locks.
- Exclusive: Locks are not compatible with any other locks.

Apart from lock types, there are transaction isolation levels for managing security among transactions:
- READ UNCOMMITTED
- READ COMMITTED
- REPEATABLE READ
- SERIALIZABLE

SQL Server has some locking optimizer hints along with lock types:
- NOLOCK
- HOLDLOCK
- UPDLOCK
- TABLOCK
- PAGLOCK
- TABLOCKX
- READCOMMITTED
- READUNCOMMITTED
- REPEATABLEREAD
- SERIALIZABLE
- READPAST
- ROWLOCK

Describe in brief SQL Server locking.

SQL server has a locking mechanism which locks the resources to be used by transactions. This is an automatic mechanism. Different modes are used for locking. Locking at a lower granularity would mean locking rows while locking at a higher granularity would mean locking tables.