The first, and most important, choice is the type of tip you need, the four mains choices are: Slotted, Pozi, Phillips or Torx . There are other more specialised screwdrivers such as Nutspinners plus a wide range of security bits available.
Slotted (or Flat Head) With slotted screwdrivers it is important to ensure that the tip matches the width of the screw slot, otherwise you run the risk of damaging the screw head. Slotted screwdrivers are available with either Flared or parallel tips. For the majority of uses the flared tip is ideal but if the screw needs to be driven below the material surface a parallel tip screwdriver should be used.
Phillips (or Cross Head) The Phillips driver has four simple tapered slots cut out of and was designed so that with a Phillips tip could turn a Phillips screw with increasing torque until the torque gets to be too strong for the tip then the screwdriver tip winds itself out of the screw thus preventing over-tightening and or damage to the screw head. The main disadvantage of Phillips screws is that they are visually quite similar to Pozidriv, thus many people are unaware of the difference or do not own the correct drivers for them, and use incorrect screwdrivers. This results in difficulty in unscrewing the screw and in damage to the slot, rendering any subsequent use of a correct screwdriver unsatisfactory.
Pozidriv (Or SupaDriv) It is similar in appearance to the classic Phillips cross-head, but in fact is very different. The pozidriv has four additional points of contact, and does not have the rounded corners that the Phillips screw drive has. The biggest advantage it offers is that, when used with the correct tooling in good condition, it does not cam out, allowing great torque to be applied. The pozidriv screw can easily be distinguished by a line embossed in the screw head at 45 degrees to the slots for the driver.
Torx (Or Star Bits) By design, TORX head screws resist cam-out much better than Phillips or slotted screws. Where Phillips heads were designed to cause the driver to cam out, to prevent over-tightening, TORX heads were designed to prevent it and to achieve a desired torque consistently. TORX screws are commonly found on vehicles, computer systems and consumer electronics, but are also becoming increasingly popular in construction. TORX head sizes are described using the capital letter "T", followed by a number. A smaller number corresponds to a smaller point-to-point dimension of the screw head. Only the proper driver can drive a specific head size without risk of ruining the driver or screw.