OEM stands for Original Equipment Manufacturer. These are the big brands like Dell, Acer, Toshiba, etc. They are large enough that they have motherboards and other components customized for them. Especially the BIOS features. OEMs don't want many options in the BIOS. Basically they want one size fits all. OEM can also be applied to hardware components and versions of Windows.
There are only a handful of actual motherboard manufacturers. These manufacturers may produce motherboards with their own brand name on them and also manufacture motherboards to the specs of the companies mentioned above.
Shops refers to computer shops or computer repair shops.
In response to your #23 response, Tyan is a company that specializes in server class motherboards. The motherboard in question here predates the invention of multi-core processors. So, to get more work out of one computer they have TWO processors.
The reference to 1-1.2GB is the range of processors that will work on that board. You need to remember that not too long ago that a 1GB processor was the brass ring, so to speak. I don't keep up with server class motherboards but I will state that the line between server class and even consumer class, has blurred. It is not about raw power anymore. Server class machines still need reliability and usually lots more memory. More dependable memory at that. Server class also usually need lots of disk storage with dependability. Dependability means redundancy. That can be in the form of a RAID array with the ability to hot swap hard drives without shutting down. They may also have redundant power supplies.
Hope that more than answers your questions.