What are Isolation Levels? What are the differences?

An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation level is Read Committed. Here are the other isolation levels (in the ascending order of isolation):

Read Uncommitted, Read Committed, Repeatable Read, and Serializable.
SET TRANSACTION ISOLATION LEVEL, which lets you customize the isolation
level at the connection level.

Read Uncommitted
A transaction operating at the Read Uncommitted level can see uncommitted changes made by other transactions. At this level of isolation, dirty reads, nonrepeatable reads, and phantoms are all possible.

Read Committed
A transaction operating at the Read Committed level cannot see changes made by other transactions until those transactions are committed. At this level of isolation, dirty reads are not possible but nonrepeatable reads and phantoms are possible.

Repeatable Read
A transaction operating at the Repeatable Read level is guaranteed not to see any changes made by other transactions in values it has already read. At this level of isolation, dirty reads and nonrepeatable reads are not possible but phantoms are possible.

Serializable
A transaction operating at the Serializable level guarantees that all concurrent transactions interact only in ways that produce the same effect as if each transaction were entirely executed one after the other. At this isolation level, dirty reads, nonrepeatable reads, and phantoms are not possible.