What is SQl injection ?

It is a Form of attack on a database-driven Web site in which the attacker executes unauthorized SQL commands by taking advantage of insecure code on a system connected to the Internet, bypassing the firewall. SQL injection attacks are used to steal information from a database from which the data would normally not be available and/or to gain access to an organization’s host computers through the computer that is hosting the database.
SQL injection attacks typically are easy to avoid by ensuring that a system has strong input validation.
As name suggest we inject SQL which can be relatively dangerous for the database.
Example this is a simple SQL
SELECT email, passwd, login_id, full_name
FROM members
WHERE email = 'x'

Now somebody does not put “x” as the input but puts “x ; DROP TABLE members;”.
So the actual SQL which will execute is
SELECT email, passwd, login_id, full_name
FROM members
WHERE email = 'x' ; DROP TABLE members;
Think what will happen to your database.