A sandal is a shoe with an open top. They don't have shoelaces, but are usually latched together with either a belt-like strap, or velcro.
A flip flop is like a sandal, but usually much less sturdy, and more bendy, or "cheap". You can just slip them right onto your foot.
A slipper is another slip-on shoe that is used for comfort, they are usually furry or squishy on the inside. You usually do not wear slippers outside, at the store, etc, mostly just for comfort inside your home. :)