What is SQL Injection ?
SQL injection has evolved from two major words - SQL & Injection.
SQL is the Structured Query Language that stores the data . Injection is form of an attack to inject or insert any malicious or non-malicious data without authorisation.
Now, Why it's so famous, Well, There used to be lot many application and Website which were and still are vulnerable to this attack.
It has been ranked no.1 attacking method for websites since 2010 by OWASP. Google Search Results for: OWASP
Now, If you Are Web Developer than, this is must Read Page for you, this will tell you what stuff makes it vulnerable, and how can you fix them.
As per Wikipedia ,
SQL injection is a code injection technique, used to attack data-driven applications, in which nefarious SQL statements are inserted into an entry field for execution (e.g. to dump the database contents to the attacker).