Most envelopes are either wallet or pocket style.
A wallet is where the flap is on the long edge,
and a pocket is where the flap is on the short edge.
There are also a few different standard sizes that are commonly used.
These are designed to take the following sizes:
C4 – A4 flat
C5 – A5 flat, A4 folded once
DL – 1/3 A4 flat, A4 folded twice
C6 – A6 flat or A5 folded twice
The adjacent image shows examples of these different envelope types.