When choosing a new set of wheels you'll need take several specifications into consideration, and those specifications can appear daunting at first glance, so here's a simple guide...
Example: 7x17 4/114.3 ET50 64
||Width and diameter (in inches).
||Stud pattern (ie. amount of bolt/stud holes the wheel has - most cars have either 4 or 5).
||PCD (Pitch Circle Diameter). To obtain this measurement (see diagram), draw a circle which passes through the centre of each wheel stud/bolt hole. Now draw a line from the centre of a wheel stud/bolt hole to the opposite side of the circle (crossing through the centre of the circle) - The distance of that line is the PCD (in mm).
||Offset (in mm). This is the distance between the centre-line of the wheel and the mounting surface. The offset can be either positive, zero, or negative (see diagram). If the mounting surface is exactly in-line with the centre-line of the wheel then the offset is zero. If the mounting surface is between the centre-line and the 'face' of the wheel then the offset is positive. And of course, if the mounting surface is between the centre-line and the 'rear' of the wheel then the offset is negative.
It's important that your wheels have the correct offset otherwise you may find the tyres will rub against the inside wheel arch, or extrude beyond the outer wheel arch.
||Centre bore (in mm). This is the diameter of a bore that is drilled or cast into the centre of the wheel. The bore should be the same diameter as your hub (aka 'Hub-centric') because it is mounted onto it - This allows the hub to take the full weight of the car instead of the bolts/studs. If the centre bore size of a wheel is too big then an alloy or plastic ring can be used to effectively reduce the diameter of the bore. If your wheels are not 'hub-centric' then you will experience vibrations through the steering wheel.