This is a standard every-day envelope with many uses as well as for greetings cards. It takes an A4 sheet of paper folded twice (down to A5, then to A6) (sometimes called a French Fold). A6 is the size of many postcards, correspondence cards, and such like.
Greetings cards envelopes invariably have a diagonal gummed flap, and, as a disposable item, can sometimes be made of thin paper. Good C6 envelopes use paper of 90gsm, or up to 110gsm. They are often available in a wide range of colours.