The snapshots can potentially destroy your save games, even though we try our best not to do that. Only use them if you know what you are doing. To “install” a snapshot, you download the JAR file and replace the one located in your “application data” folder. The easiest way to find that folder is to start the game, select “texture packs” and click on the “open texture pack folder” button. It will locate “.minecraft/texturepacks”, and the JAR file should be placed in “.minecraft/bin”.
To UNINSTALL the snapshot, start the launcher (Minecraft.exe on Windows) and click the “options” button. Then click on “force update” and login as usual. The launcher will then download the official JAR file for you. Note that save games created in the snapshot may not work (and probably doesn’t) in the official version of the game.