What is ACID fundamental? What are transactions in SQL SERVER?

The transaction is a sequence of operations performed as a single logical unit of work..The logical unit of work should exhibit 4 properties, Known as the ACID (Isolation , Atomicity, Consistency, and Durability) properties, to qualify for the transaction:

1)Atomicity:

The transaction should be an atomic unit of work; either all of its data modifications are performed or none of them is performed.

2)Consistency:

When done, a transaction should leave all its data in a consistent state. In a relational database, all rules should be applied to the transaction's modifications to maintain all data integrity. All internal data structures, like B-tree indexes or doubly-linked lists, must be correct at the end of the transaction.

3)Isolation:

The Modifications made by concurrent transactions should be isolated from the modifications made by any other concurrent transactions. A transaction either see data in the state it was before another concurrent transaction modified it, or it views the data after the second transaction has done, but it does not see an intermediate state. This is known as serializability as it results in the ability to reload the starting data and replay a series of transactions to end up with the data in the same state it was in after the original transactions were performed.

4)Durability

After the transaction has done, its effects are permanently in the place in the system. The changes persist even in the event of a system failure.