OEM is the same as dealer parts as dealer parts are also OEM.
If you purchase OEM parts they are dealer parts.
Aftermarket parts are any parts that are not made by the manufacturer.
The difference between OEM and genuine parts is a genuine part is a part that is supplied by the vehicle manufacturer in their packaging.
An OEM part or Original Equipment part is a part that is made for them to their specifications but from an external company.
OEM parts are better than aftermarket parts in most cases and for some parts.
However some aftermarket parts are just as good as OEM parts and are cheaper.
It is OK to use non OEM parts although if your vehicle is still under warranty the use of non OEM parts can void your vehicles warranty.
However if your car is not under warranty or you have the car paid off there's nothing wrong with using aftermarket non OEM car parts which can be just as good as OEM parts and cheaper.
However electrical parts are better when bought as OEM as they last longer than non OEM electrical car parts.
The best brand of aftermarket car parts is Dorman, Bosch, Spectra and Delphi.
Duralast auto parts are also good aftermarket car parts.
It is better to buy car parts online if you don't need the car parts right away.
If you need the car parts right away then it's better to buy them at an auto parts store but if you can wait for the car part then it's best to buy the car parts online as they can be cheaper than buying car parts in store.
The best place to buy auto parts online is RockAuto, eBay, Partsgeek.com and Amazon.com
These are the places where I buy my auto parts from.
Most auto parts I buy from eBay although sometimes I can only find the auto parts I need at RockAuto, Amazon or Partsgeek.
You can get most auto parts online cheaper than you can in an auto parts store.