White business envelopes
These are usually thicker (hence stronger) than Greetings Card envelopes, and do not have diagonal flaps. The flap, if on the long edge, is called 'Wallet' style', and on the short edge 'Pocket' style). They invariably follow the ISO 'C' series of sizes, with the addition of the DL size (for one third A4 sheets).
UK Royal Mail has a charging structure to ccommodate on the C5 and C4 sizes.
They can have windows cut into them. If the addressee is written on the letter inside it does not have to be repeated on the outside of the envelope. (This can save errors when stuffing multiple letters into a pile of plain, but addressed, envelopes. It is easily done...we know from experience!)