These are really good quality C4 envelopes with a peel and seal closure. They are made of bright white paper, which is thick enough to get through the postal system without being torn to pieces. The paper is not tear-proof, but it is thick enough to withstand (in my experience) the battering Royal Mail can inflict, along with some exposure to rain during delivery. I no longer feel the need to reinforce seams with sellotape and feel confident that documents will arrive in one piece at the other end.
There are cheaper envelopes around but if you want to make a good impression with the recipient and ensure that your letters and documents reach the other end safely and in good order, these are just the thing.