What's the difference between theorem, lemma and corollary? Can anybody explain me what is the basic difference between theorem, lemma and corollary? 
We have been using it for a long time but I never paid any attention. I am just curious to know.

Here is some information from this link:

Theorem — a mathematical statement that is proved using rigorous mathematical reasoning.  In a mathematical paper, the term theorem is
  often reserved for the most important results.
Lemma — a minor result whose sole purpose is to help in proving a theorem.  It is a stepping stone on the path to proving a theorem.
  Very occasionally lemmas can take on a life of their own (Zorn’s
  lemma, Urysohn’s lemma, Burnside’s lemma, Sperner’s lemma).
Corollary — a result in which the (usually short) proof relies heavily on a given theorem (we often say that “this is a corollary of
  Theorem A”).

