I have only been decorating for a short while and have been making my own icing. Basically, what I'm wondering is in regards to buttercream frosting. Where it normally has butter and often milk in it, shouldnt it always be in the fridge even when on the cake? Wont it go back on the cake if not kept cold?

Also, how long can I store un used icing in the fridge?

Hi and Welcome to CC, ctucker. L)

Decoding CC acronyms:

http://forum.cakecentral.com/cake-decorating-ftopict-2926-.html

Sugar is hygroscopic (or moisture absorbing) so it can be used to alter the "water activity" in foods.

The large proportion of powdered sugar in American b/c's controls the water activity in the butter and/or milk or cream so that it doesn't require refrigeration.

Most American b/c's are fine just stored in a tupperware container on the counter. However, unused American b/c can be stored in the fridge for quite a while or even frozen for future use.

Overview of different icings by type:

http://tinyurl.com/yh44gu
(Has handy frosting troubleshooting chart on last page.)

HTH