From Wikipedia: Foley is the reproduction of everyday sounds for use in filmmaking. These reproduced sounds can be anything from the swishing of clothing and footsteps to squeaky doors and breaking glass. It helps to create a sense of reality within a scene.
Bees: recreate the sound of bees by recording these stupid assholes who race gas-powered remote controlled boats around the lake by my house.
Gunshot: to recreate the sound of a gunshot, how about you get up from your desk, quit being a pussy, and shoot that asshole you’ve always been talking shit about.
Sex: recreate the sounds of sex by holding one end of a raw hot dog and tapping the other end against your inner thigh over and over.
Sex (alternate): same procedure, but first drain a can of baked beans and slather the juices on your thigh.
Door opening: how about that same goddamn fucking sound everyone uses from Doom? Seriously, every movie uses this same stupid door closing sound from a computer game. Just watch the first six seconds of this video and prepare to have every movie completely ruined from here on out as you start hearing it everywhere.