Clash was introduced in the Lorwyn block. Initially it seems like a random effect and thus is not ideal for competitive play but it is more useful as a deck fixer and making your draws smoother.

701.18. Clash

• 701.18a. To clash, a player reveals the top card of his or her library. That player may then put that card on the bottom of his or her library.
• 701.18b. "Clash with an opponent" means "Choose an opponent. You and that opponent each clash."
• 701.18c. A player wins a clash if that player revealed a card with a higher converted mana cost than all other cards revealed in that clash.

Rules Translation

• 701.18a. - This explains how exactly you clash. You both reveal the top card of your library and then you may choose to put it on the bottom of your library.
• 701.18b. - This is the legalese in the rules explaining how exactly you clash.
• 701.18c. - This is how you win the clash.