The locks which get applied on various resources of SQL server can be classified into below models :

• Exclusive (X): This lock type, when forced, will guarantee that a page or row will be available only for the transaction that forced the lock, as long as the transaction is running. The X lock will be forced by the transaction when it needs to manipulate the page or row data by DML operations like inserting, modifying and deleting. This lock can be forced to a page or row just if there is no other shared or exclusive lock forced as of now on the resources. This ensures that only one exclusive lock can be applied and no other lock can be applied afterward till the time the previous lock gets removed.
• Shared (S): This lock type, when forced, will save a page or row to be accessible just for reading, which implies that some other transaction will be not allowed to manipulate record till the time lock remains active. Nonetheless, a shared lock can be forced by multiple transactions CONCURRENTLY over a similar page or row and in that manner, multiple transactions can share the capacity for data reading. A shared lock will permit writing tasks, yet no DDL changes will be permitted
• Update (U): an update lock is like an exclusive lock however is INTENDED to be more adaptable. An update lock can be forced on a record that as of now has a shared lock. In such a case, the update lock will force another shared lock on the intended   When the transaction that holds the update lock is prepared to change the data, the update lock (U) will be changed to an exclusive lock (X). While the update lock can be forced on a record that has the shared lock but the shared lock can't be forced on the record that as of now has the update lock
• Intent (I): The idea behind such a lock is to guarantee data modification to be executed appropriately by stopping another transaction to gain a lock on the next in the hierarchy object. Generally, when a transaction needs to obtain a lock on the row, it will gain an intent lock on a table, which is a higher chain of the intended object. By obtaining the intent lock, the transaction won't enable other transactions to procure the exclusive lock on that table.
• Schema (Sch): This lock is applied on a table or index when we want to manipulate any changes in that resource. We can have only one Schema lock at a given point of time. This lock gets applied when we perform operations that depend on the schema of an object.
• Bulk update (BU): This lock is required when bulk operations need to perform. At the point when a bulk update lock is GAINED, different transactions won't most likely access a table during the mass load execution. Be that as it may, a bulk update lock won't avoid another bulk update to be executed in parallel.

Locking hierarchy of database objects  can be understood by the below diagram and lock is ALWAYS obtained from top to bottom:

Below is the lock compatibility matrix between different modes of lock available: