Genuine Bike Parts
Genuine parts are supplied by the bike manufacturer, usually through it's dealer network. Many of these parts are made by the bike manufacturer but a lot more are made by other companies on their behalf (see OEM parts below)
Original Equipment Manufacturers or OEM Motorcycle Parts
The motorcycle manufacturers don't manufacture every part of their bikes. They often use parts that are designed and manufactured by independent OEM's. If you take a look over your motorbike you will see many examples of this by just searching for other brand names such as Tokico & Nissin brake callipers. Nippon Denso clocks and instruments. Showa, Ohlins, Sachs, FVQ suspension. Keihin and Mikuni carbs etc etc. Some of these parts are exclusive to a particular model but many are generic. When you buy these parts packaged by the bike manufacturer you will be paying a premium over the same part sold as OEM.
Pattern Motorbike Parts
Pattern parts are copies of a genuine or OEM part. Often pattern parts are produced because a direct replacement is no longer available for a particular model. The quality of pattern parts is variable with many parts exceeding the quality of the originals due to improvement in manufacturing processes, however some parts are produced that are clearly inferior and these must be avoided as they are prone to failure. Typically pattern parts are much cheaper than original parts and as long as you are careful to only buy quality pattern parts there is no reason they should fail prematurely and you will be rewarded with a bargain.
Aftermarket Motorcycle Parts
Aftermarket or accessory parts are parts that specifically offer something extra above and beyond the standard parts. These parts are usually of a different design to try to improve or individualise a bike to suit the owner. Some of these parts are engineered to improve the quality by reducing weight, improving ride, adding strength, modifying power characteristics or just simply looking better. Some achieve this with the use of exotic materials such as carbon fibre, Kevlar, titanium alloys, billet aluminium etc. Others make more use of specialist engineering knowledge or simply better design.