Some of the chess rules are lesser known to beginners than others. Can anyone suggest such rules, like En Passant, Three-fold repetition, Fifty-move rule, Stalemate, Castling Problems?
I'm going to go out on a limb and suspect that when you say "rules are not much popular", you mean "rules that are lesser known".
I can think of two rules which beginners are known to struggle with: castling and en passant.
The concept behind castling is relatively simple, but beginners commonly struggle with the conditions that all must be "true" in order to castle successfully, which include never having moved your king and your king not walking through check to arrive to his final position.
From my experience, en passant is much more difficult for newbies to grasp. It is a special type of pawn capture where your pawn takes your opponent's pawn "in passing". Details about this rule can be found here: http://en.wikipedia.org/wiki/En_passant