Dirty Read: When one transaction is try to read a record , which is the part of half finished transaction.
Lets understands it with example
* A and B are two people working on Employee Table
* B create a huge transaction and update the salary of Employee y to 7000 as a part of transaction.
* In the mean time A select the record of employee y.
* A find the salary of y is 7000.
* Some how transaction of B gets rollback, and salary of y is again set to 2000.
* Now you can see A has wrong salary of Y.
* This this is known as Dirty Read.