A reverse shows a strong hand with the first bid suit always being longer than the second. A reverse means that partner has to bid to the 3 level in order to return to opener's first suit.
2♥ is a reverse because if South wants to go back to North's first suit it will need to be at the 3 level with a bid of 3♦.
2♣ is not a reverse. South could bid 2♦, returning to opener's suit at the 2 level.
Why can't a reverse be made with two 4 card suits? In most auctions if a player shows two suits, the hand should be unbalanced. If you're balanced you're either going to open 1NT or bid NT on the second round. If you haven't bid notrumps by the second round you won't be balanced. The first suit will, therefore, always contain at least 5 cards.
Reverses are forcing to game so you have to keep bidding.
On the following hand North has reversed showing 5 clubs, 4 diamonds and a strong hand.
You have a club fit.
This time you are stronger and it's up to you to indicate your extra strength.
You have spades covered, bid 3NT.