Why Bring one? These are so you don't get your pad or sleeping bag wet or full of holes. If you have a big enough tarp, you can use it as a shelter as well. You just fold the tarp over your sleeping bag.

What kind should I get? I use a footprint tarp. It is a tarp designed to fit the bottom of my dad's tent perfectly. I just use it as a tarp. You can also just use a sheet of construction plastic or a regular blue tarp. These also double as shelters if you can find a way to make them work