Lock SQL Server ...3

--> Update
Update locks (U) are acquired just prior to modifying the data. If a transaction modifies a resource, then the update lock is escalated to an exclusive lock, otherwise it is converted to a shared lock. Only one transaction can acquire update locks to a resource at one time. Using update locks prevents multiple connections from having a shared lock that wishes to eventually modify a resource using an exclusive lock. Shared locks are compatible with other shared locks, but are not compatible with Update locks. Update locks are also used for inserts into a table with a clustered key.
--> Exclusive
Exclusive locks (X) completely lock the resource from any type of access including reads. They are issued when data is being modified through INSERT, UPDATE and DELETE statements. You should try to minimize the time during which any resource is locked exclusively by making your data modifications as short as possible. Exclusive locks are usually the cause of blocking in the applications.
--> Schema
Schema modification locks (Sch-M) are acquired when data definition language (DDL) statements are being executed. This includes modifying tables or views through adding columns, dropping columns, adding or dropping constraints, etc. Schema stability locks (Sch-S) are acquired when queries are being compiled; these will not block any other types of locks, including exclusive locks. Therefore, schema locks will not cause any blocking. However, when queries are compiled, the tables affected by the queries cannot be altered through DDL statements.
--> Bulk Update
Bulk update locks (BU) are used when performing a bulk-copy of data into a table with the TABLOCK hint. These locks improve performance while bulk copying data into a table; however, they reduce concurrency by effectively disabling any other connections to read or modify data in the table. Another way of acquiring BU locks is by specifying the "table lock on bulk load" option with the system stored procedure sp_tableoption.