ACID

Atomic, a transactions is all or nothing, if one part of a transaction fails then the entire transaction fails.
Consistent, the database will go from one valid state to another.
Isolation, concurrent execution of transactions result in a system state that would be obtained if transactions were executed serially.
Durability, Once a transaction has been commited it will remain so.

Transaction Isolation levels

These are levels that define how isolated the Isolation property of a transaction is. Note they are transactions specific, different transactions can have different isolation levels.

Serializable

Statements can not read data that has been modified but not yet committed by other transactions.
No other transaction can modify data the has been read by the current transaction unitl the current transaction completes.
Other transactions cannot insert new rows with key values that would fall in the range of keys read by any statement in the current transaction.

Repeatable read

Statement can not read data that has been modified but not yet committed by other transactions ( No dirty reads ).
No other transaction can modify data the has been read by the current transaction.

Note You can get phantom rows as inserts from other transactions are allowed.

Read committed

Statements can not read data that has been modified but not commited by other transactions ( No dirty reads).

Note Data can be changed by other transactions between individual statements within the current transaction resulting in nonrepeatable reads ( Phantom Data ).

Read uncommitted

Statments can read rows that have been modified by other transactions but not yet committed.

References

https://en.wikipedia.org/wiki/ACID
https://msdn.microsoft.com/en-us/library/ms173763.aspx