Posidriv is a four-flute fastener drive that's less prone to cam-out than is Phillips. Posidriv drivers and recesses look very similar to Phillips, but there is a difference, and the drivers should not be used interchangeably. Posidriv recesses are marked for identification. Pictured below are two screw heads.
The screw head at the left is Posidriv; at the right is Phillips. Note the radial strokes at the corners of the Posidriv recess. Those are there to indicate that it's a Posidriv recess.
The difference between the two drives lies in the shape of the flutes. Phillips flutes taper inwards slightly. Pozidriv flutes' opposite sides are parallel to one another. Here's a view of the two drivers.
The Pozidriv bit is the one at the left. Note how the manner in which the Pozidriv flutes are ground differs from the Phillips bit at the right. The Pozidriv flute sides are parallel; the Phillips flutes are tapered.
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