A gummed envelope is the traditional style envelope with a water soluble gum strip that you lick to seal the envelope. Gummed mailing envelopes also have the same water soluble gum strip, but the flap is specially designed to go through a mailing machine, so the machine can seal them. Self seal envelopes are known by a lot of names: peel & seal , strip seal , peel & stick, zip seal , or simple seal. They're a popular and easy-to-use alternative to traditional envelopes because, unlike gummed seals , self seal envelopes do NOT need to be moistened. (No licking! No glue sticks!)