A piece of cake

It's a piece of cake. That will be a piece of cake.

This refers to something that is easy.

Submitting a question is a piece of cake, just click "Submit your Question" on the side-bar under "Navigation" and enter it in the "comments" window.

Sometimes people say "Piece of cake" as a whole sentence, without the "a", but if any other words come before the phrase,you should use the "a".

Can you do all of these things?

Sure. Piece of cake.

"It's a" is implied.

"It's piece of cake" would be wrong because you need an "a".

Sometimes people use even shorter ways of saying it.

It's cake. Cake.

In this case we use no "a". It is not a cake (a whole cake), it is the substance,the uncountable noun, cake. English uses the same word for both. The countable "piece of cake" is like the countabe "easy task", and the uncountable "cake" is like the uncountable "easy work".

This usage is very casual.

Sometimes other words are added in. One very casual example would be the following.

It's total cake.

This means it is very easy, or even extremely easy. It seems like slang.

"Cake" is also used as an adjective in slang.

It's a cake job.

It's an easy job.