Rook
is the most valuable piece. Its worth more than even two cannons, a cannon and a horse or two horses. Valuation may differ, if you give a rook for two very active minor pieces.

Cannon and Horse are the minor pieces and are more or less equal in strength, but their strength differs during the game. In the opening and middle game the cannon is the stronger piece, but in the endgame it becomes weaker because there are less piece on the board to support the cannon. In the endgame a horse is the stronger attacker, while a cannon in many cases is the better defender. Combination of horse and cannon is stronger than two cannons or two horses.

Pawn
Is usually not even worth a tempo in the opening, but may easily win a game in the endgame. Approaching the palast the pawn becomes stronger and stronger and loses strength on the base line.

If you lose your guards, your defence becomes very weak against rooks and horses, while a lack of elephants makes it weak against cannons. **This last quote in very general terms, of course it always depends on the position.**