May I know what SQL is actually? I was told by many providers that SQl was meant for database. But "database" was very general and i don't get what they mean. I hope someone could actually tell me the details (functionality) of what SQL can do.

This kind of question is best left to you - search Google for "SQL" and go to town.

SQL - What is it
http://www.dcs.napier.ac.uk/~andrew/sql/0/w.htm

Philip Greenspun's SQL for Web Nerds - http://www.arsdigita.com/books/sql/