Envelopes are folded paper or card containers, typically used to enclose letters, cards, or other documents for mailing. They come in various sizes, styles, and materials, such as paper, plastic, or even padded envelopes for added protection. Envelopes usually have a flap that can be sealed with an adhesive strip, glue, or by moistening to secure the contents inside. They play a crucial role in communication and mailing, providing a means to protect and transport documents securely through postal or courier services. Envelopes are also commonly used for sending invitations, greeting cards, and official correspondence.