Thomas H. Huxley, a well known English religious skeptic, invented the term Agnostic in the mid 19th century (sources differ about the exact date). He combined "a" which implies negative, with "gnostic" which is a Greek word meaning knowledge.
An Agnostic can refer to one of many religious beliefs. Normally it is used to refer to someone who believes that either there is not enough evidence to discredit the existence of a god but there is also not enough evidence to prove a god.
Agnostic Atheist: An Atheist who doesn't claim to know for sure that there is no god but chooses to believe that there is most likely no god.
Agnostic Theist: A Thesit who doesn't claim to know for sure that there is a god but chooses to believe so.
Strong Agnostic: An Agnostic who thinks that the very idea of god's existence is a problem to big for the human mind to ever find a solution to. Charles Darwin may be the most famous "Strong Agnostic"
Weak Agnostic: An Agnostic who claims that there is not enough evidence to decide if a god exists or not but more evidence may prove or disprove it's existence in the future.