I was wondering what the difference is between the two, and what is one used for that the other is not or cannot be used for? TIA :)

A crusting b'cream will dry to the touch - usually within 1 hr or less.  That means you can touch a finger to it and NO icing comes off on your finger.  Crusting uses less fat/grease/Crisco/butter than non crusting

A non-crusting will never dry :)

As for use - basically they are interchangeable UNless you need to touch the cake for any reason - as in applying special decorations, move one tier on top or next to another, etc.  Then non cursting will not work as many people smudge it when they work.  Trying to smooth a smudge on non crusting ususally only makes things worse :(

Both crusting and non-crusting buttercreams are very workable. They just require different techniques. I have used both and much prefer non-crusting both in workability/smoothing and taste.  I love working with Italian Merengue Buttercream (IMBC). It holds flavors well, colors nicely, and smoothes beautifully. When you get the hang of working with it, smoothing smudges and mistakes are easy. I also think IMBC is a much better buttercream for wedding cakes as it is not super sweet like most crusting buttercreams. Many people only work with crusting buttercream for their ease of use. I would recommend that you try different kinds of buttercream and see what works best for you. 

AIMBC is the best.

Thank you, ladies, very helpful. So, the difference is... not much, really..... The IMBC, is there a standard recipe that you use, or is it your very own? Please let me know, for I would love to try it sometime :)

I really like the IMBC/mousseline buttercream recipe on Baking 911 website. It also has photos and instructions on how to make it. It can be a little difficult at first, but when it comes together, it is wonderful! The only adjustment I made is with boiling the sugar syrup. I live at 7000ft, so I only boil the sugar to the soft ball stage. Any more than that and the sugar just becomes chunks of hard candy instead of syrup.  I hope you give it a try. I know I'll never go back to crusting american buttercream! Good Luck!

http://*********.com/frosting-icing-etc/buttercream-meringues/italian-meringue-or-mousseline-buttercream-or-imbc

AI have always thought non crusting bcs much easier to work with. I have never figured out how to smooth a crusting bc!

jaja, I'm going to try it out sometime, and if I remember, I'll let ya' know how it turns out. Thanks for the info :)

..........leah said: ....... never figured out how to smooth a crusting bc!.........

Why, it's smoothed exactly the same as a non crusting :)  It's smoothed immeadately after applying while it is still soft and workable.  The crusting happens as it sits exposed to air.  Once it has crusted/dried (one can touch it w/a finger and NO icing comes off) you can use a paper towel to further smooth if there are any bumps, creases or other blemish.  Just lay the paper towel, wax paper or (some even use) typing paper on the crusted icing, lightly rub your hand OR a fondant smoother over and remove the paper - you should have a great looking, smooth finished icing.  One word of caution:  if you have waited too long (such as overnight) the 'smoothing' might cause the icing to look wrinkled or anything but smooth.  This 'finish step' needs to be done soon after it has dried on the surface while the inside is still pliable :)

kakeladi, thank you for that advice as well - very helpful if/when I use CBC :)

Thanks, Kakeladi.  Been there done that.  Moved on.