Stationery is a general name given to paper and office supplies such as envelopes, notepads, pens, pencils, paper clips, staples, etc and refers to all products sold by "stationers". The term "stationery" is frequently used to refer more specifically to paper used for written correspondence, usually decorated and/or personalized with matching envelopes.
Although it is a common error to mis-spell "stationery" as "stationary" and vice versa, the word "stationery" was originally spelled with an "a" in English. It derives from the fact that such products were sold in "stationary" shops and not from travelling vendors.