Just as ices said it is sooo yummy and you can do so many things with it, I don't always frost with Bettercream, depending on color/decoration, but I almost always make my fillings with it and everyone just loves it.
I buy frozen cartons of liquid vanilla bettercream (or Frostin Pride or Pastry Pride, I find it all to be comparable) at my local cash & carry, they are also called smart & final in some places, it's a restauraunt supply store open to the public. I don't like the pre whipped stuff from Sams, because I add whatever flavors I want to the liquid and it whips up awesome!
Melvira is the resident Bettercream expert and turned a whole ton of us on to it a while ago, here's a link to a huge post all about bettercream and I'm inserting her description below. Try it you will like it! It does not take color well though, so be prewarned, you will only get pastels if that's a concern.
Rich's Bettercreme (note the spelling... it makes a big difference in getting the right product!) is a liquid that you can buy that whips similar to heavy whipping cream. But, it is a stable icing used for cakes and cupcakes. In other words, it won't break down like whipping cream does. You can usually find it in cake stores, sometimes in restaurant supply stores, some locations even carry it at Sam's club, but not all locations have it. You can get it in a pre-whipped variety, but it's not as good, although it works if you can't get the liquid. The pre-whipped seems to be easier to get your hands on in some areas.
For more information, go to www.richs.com
and find Bettercreme under their products guide!