A "crumb coat" is a thin layer of frosting that seals in the afore mentioned crumbs. It need not be perfectly smooth or particularly even, it just catches the crumbs. The crumb coat is also a must for sculpted cakes. I usually pop a cake into the freezer after it has been baked and cooled to room temperature (Wrapped first in plastic wrap and then in aluminum foil). A firm cake is much easier to carve. After it has firmed up a bit in the freezer, I unwrap it, and carve according to my plan. I usually have a drawing or picture to work from. Here is Hello Kitty's crumb coat:
You can see all the cake crumbs and even some of the cake, but it's ok! After the crumb coat has been applied, return the cake to the fridge to firm up the frosting, in this case it's my buttercream frosting.
If this cake was going to be covered in fondant, this is the point when the fondant would be applied. However, for Hello Kitty, I applied the second layer of frosting and found that I could still see the cake in a couple of spots, so I popped it back into the fridge, and applied more frosting where it was necessary. The trick to smoothing out frosting evenly, is using an off set spatula that you have run under HOT water. The hot water slightly melts the butter in the frosting and easily smooths out the frosting. Return the spatula to the hot water after every couple of passes on the cake and keep a towel handy for drying it off after you run it under the water.
I made Kitty's features and bow out of fondant and found some thick black licorice for her whiskers. The pink sprinkles set off her smooth white frosting perfectly (and cover the cake board as well)!
While it's nearly impossible to achieve the perfectly smooth look of fondant with frosting, it can come pretty close.
Here's Hello Kitty boxed up and ready to feed a gaggle of six year old girls. Doesn't she look a little sad to be trapped in the box?