What is the difference between an opening and a defence? I would like to know why some openings are called "defences" and other all called "openings." What is the difference?
"The opening", in general, is the first phase of a game of chess, in which both sides develop their pieces. It is followed by "the middlegame" and, if the game gets that far, "the endgame".

When people talk about "an opening", they are referring to a particular series of opening moves that has a name. For example, 1.d4 d5 2.c4 e6 is the Queen's Gambit Declined, and 1.d4 d5 2.c4 dxc4 is the Queen's Gambit Accepted.

Openings that are defined by the moves that Black makes are often called "defenses", since White is generally considered the attacker and Black the defender by default. For example, 1.e4 c5 is the Sicilian Defense and 1.e4 e6 is the French Defense.