There are 3 kinds of locks in SQL Server,

 

1. Shared locks:  They are used for operations which do not allow any change or update of data. For example, SELECT.

2. Update locks: They are used when SQL Server wants to modify a page. The update page lock is then promoted to an exclusive page lock before actually making the changes.

3. Exclusive locks:  They are used for the data modification operations. For example, UPDATE, INSERT, or DELETE.