What kind of IKEA screw is this?
That's a Pozidriv head that might have been chewed up a little by the inappropriate use of a Phillips screwdriver.


Public Domain Image

Phillips #2 on the left, Pozidriv #2 on the right.

Pozidriv screwdrivers come in three main sizes, usually named PZ3, PZ2 and PZ1 from largest to smallest. There is also a rarely seen PZ0. The most common is PZ2. You should use the largest bit that will engage the screw fully.

Here are some examples of Pozidriv screwdriver bits for interchangeable-bit screwdrivers (hand screwdrivers and power screwdrivers)



It is essential to 


Use a Pozidriv bit, never a Phillips bit
Use the correct sized Pozidriv bit.


Used correctly, Pozidriv bits are much more positive than Phillips and are far less likely to "cam-out" (lift and slip under torque).

Phillips screws often don't stand up to repeated use unless you are exceptionally careful. For use in the home, Pozidriv predominates in the UK and much of Europe. Canadian homeowners seem to prefer Robertson (square recess), others Allen (hexagonal recess). It may be that IKEA's European roots explain why it used Pozidriv for this application (although they seem very fond of Allen bolts). 



Useful resources


Wikipedia: List of screw types
Is it a bad idea to use a Pozidriv screwdriver on a Phillips head?
Why does my drill bit destroy the screw head?