Things that easily adhere to other things are sticky. Glue and tape are sticky. So is a wad of chewed gum — especially when you step in it. Yuck.
Many things are meant to be sticky, like the sticky notes you use for bookmarks or the sticky lint roller you use to remove cat hair from your pants. You can also describe hot, humid weather as sticky, or an awkward problem you can't easily shake: "They brought up a number of sticky issues in the meeting this morning." If someone has "sticky fingers," it means they're inclined to steal things.