There are lots of different car parts available and it can be confusing weighing up the different options, especially when you're buying from non-dealership sources. Purists will argue that OE (Original Equipment) parts are better, however if you're looking for the best value for money, then aftermarket parts might offer a more cost-effective solution.
OE parts are the exact car part that was fitted to your car by the manufacturer at the factory. These are the car parts that will be used by your local garage/mechanic when they're repairing/servicing your vehicle. The main manufacturers like BMW, VW, Audi, Mercedes etc don't manufacture their own parts and instead buy them from Car Part Manufacturers (eg Bosch) and assemble them into cars. These car parts are then sold to their dealership networks and also distributed independently.
The same company who builds these OE parts for the car manufacturer then produces a similar version of the part with their own brand on it. For example, if GMB build the water pump that is used in a BMW car then they will also produce their own branded version of this same water pump which they will then sell to car dealers and independent garages and supply them in their branded packaging.
Many car parts brands also offer a 'OE Quality' version of their products which are designed to be as close to the original OE parts as possible and these can often be used by garages/mechanics when fitting replacements to your vehicle. When deciding which parts to fit it really comes down to the budget that you have and also the reputation of the car parts brand, as there are some brands who produce a higher standard of OE quality than others.