Postcards are cards of a specific size (A6) with a printed stamp that are used for short messages.
Envelopes with printed stamp are a special type of postal stationery for which envelopes of a specific size and quality are used.
Aerograms are a special type of air-mail stationery made from a single sheet of folded paper.
Here you can find a database of picture cards with printed stamp. Picture cards have one entire page dedicated to a picture motif.
Postal stationeries are usually cards with a printed stamp or heading, such as postcards or picture cards.