I was just looking up for cheats, and tips on nintendogs for ds. And I kept finding things about action replay for games, especially nintendogs. What is action replay for nintendo ds and how do you get it?

Its a device that fits into your Nintendo DS game slot and edits certain values in the game with 'codes' allowing you to cheat. Its a cheat device.

As per the http://en.wikipedia.org/wiki/Action_Replay_DS

The Action Replay DS (abbreviated AS, ARDS, AR) is one of a line of products manufactured by Datel specifically designed for use with the Nintendo DS. It is used to enhance a game, usually by giving the player extreme advantages not normally obtainable. While earlier versions of the software forced users to input codes manually via the DS itself, versions 1.52 and later allows users to enter cheat codes through a PC application, which are copied to the software via USB.