If you use OR against a table when you are trying to retrieve rows on matching criteria, it will search the table as many times as matching criteria you gave. However, if you use IN, it will search the table just once and give the output on the search criteria.
Basically, instead of using this:
SELECT * FROM table1 WHERE col1='A' or col1='B' or col1='C'\
SELECT * FROM table1 WHERE col1 IN ('A','B','C')