How is different between "defendant" and "accused"?
I see both of them are used to describe a person in law but which in case we'll use "defenndant" and "accused"?

Question

Answer

In a criminal case, the two sides appearing before the judge are "the prosecution", which is a government department, and "the accused".

In a civil case, the two sides are "the plaintiff" and "the defendant".

Answer

First a person is accused of doing something against the law, so they are "charged" with breaking/not complying with a specific law.  Then, when they go to court, they are the defendant (they must try to defend themselves against the charges). So the term defendant only applies/is only used when the person who was charged goes to court.

Answer

Two useful glossaries of legal terms:

The US:

http://www.uscourts.gov/Common/Glossary.aspx

Britain:

http://www.routledge.com/cw/slapper-9780415639989/s1/glossary/