A Post-it note (or sticky note) is a small piece of paper with a re-adherable strip of glue on its back, made for temporarily attaching notes to documents and other surfaces. A low-tack pressure-sensitive adhesive allows the notes to be easily attached, removed, and even re-posted elsewhere without leaving residue. The Post-it's signature adhesive was discovered accidentally by a scientist at 3M. The original notes' canary yellow color was chosen by chance, from the color of the scrap paper available at the lab next door to the Post-it team.

Source: Wikipedia