A Post-it note (or Sticky Note) is a piece of stationery with a re-adherable strip of adhesive on the back, designed for temporarily attaching notes to documents and other surfaces.
It is available in a wide range of colors, shapes, and sizes, Post-it notes are most commonly a 3-inch (76 mm) square, canary yellow in color. A unique low-tack adhesive allows the notes to be easily attached and removed without leaving marks or residue.
It can be used as reminders. I use them regularly in my office to remind me about things I need to do :)
Read more here :
And this is how they look :