using the IN clause, you're telling the rule-based 
optimizer that you want the inner query to drive the outer 
query (think: IN = inside to outside). 

When you write EXISTS in a where clause, you're telling the 
optimizer that you want the outer query to be run first, 
using each value to fetch a value from the inner query 
(think: EXISTS = outside to inside). 

Thank you

HERE'S THE SIMPLE ANSWER:
WHEN YOU USE 'IN', WHILE CHECKING FOR WHERE CONDITION SQL 
SERVER ENGINE DOES WHOLE TABLE SCAN.
IF YOU USE 'EXISTS' AS SOON AS ENGINE FINDS THE REQUIRED 
ROW IT WILL STOP EXECUTING QUERY AND GOING FURTHER SCANNING 
TABLE. 

SO BASICALLY EXISTS IS FASTER AS COMPARED TO IN.

IN work like a OR in sql query but in is faster than OR in 
case of IN server will search whole table
EXISTS is use to find the required row  and does not search 
whole table