These Zero Insertion Force (ZIF) sockets clamp onto a chip, making a good electrical connection when you push the lever down.
When you release the lever, it releases the chip. This allows you to repeatedly insert or remove a chip from the socket without damage to either.
These are great for two situations: When you want to replace the chip that goes in a given place frequently, or when you want to program one chip after another in the same fixture. (For example, we use them to program the chips in our open-source kits!)
These 20-pin ZIF sockets (0.3" wide) are perfect for programming microcontrollers in DIP packages with up to 20 pins, including 8-pin, 16-pin, 18-pin, and 20-pin devices. There's even a little bar to indicate where to put a DIP-8 if you want to use one. They're great for programming the ATtiny2313 chips.
When using ZIF sockets, keep in mind that the pins are a bit wider than regular IC pins-- they fit in most pad-per-hole type prototyping boards, but may not fit in a chip socket or circuit board that was meant to hold a chip alone.