I know that OEM stands for original equipment manufacturer but whats the simplified definition of this? If im going to buy OEM earphones, are they like a fake/replica version?
Since OEM stands for Original Equipment Manufacturer, then, no, they would not be like a fake/replica version of anything.
For example: if you have a Ford minivan and get it serviced at a Ford dealership, they will almost certainly use OEM (Ford brand) parts to do the repairs, but if you get it done at an independent shop, they will probably NOT use Ford brand parts, unless that is all that is available.
So if you are buying OEM ear-buds to go with your iPod, you would be buying Apple ear-buds.
OEM earphones at Amazon http://amzn.to/1Ei1err
E.g. Apple supplies headphones with their iPhones, iPods & iPads. If you bought another pair like those that were included then that is OEM. Apple made the headphones & the product you use them for.