Mixed media is very closely related to other art forms such as assemblage and collage. Assemblage is a 3-dimensional sculptural form, in which found objects are "assembled" together in a quirky and unique way. Collage involves a rearrangement of pre-existing elements, such as cut-up paper of all sorts (newspaper clippings, gift wrap, vintage ephemera, old book pages, stamps, postcards, etc), as well as various other small items glued to the surface of the paper, canvas, or wood.
In general, mixed media main feature is usually a combination (or mix) of media, such as acrylics, oils, crayons, pencils, and anything else you can imagine! Mixed media is fun because it can take so many forms. The sky's the limit!