Although you could be referring to many different things, the most likely thing is a PCI card. These are PCBs with a specific interface slot that can add specialty functions to a computer. They are commonly used for network interfaces and TV tuner cards. See this page for more info:
http://en.wikipedia.org/wiki/Conventional_PCI
It is a printed circuit board that you can insert into a computer to give it added capabilities
The "primary circuit board" for a computer is called the "Motherboard". This is the board that contains the housing and sockets for all of the coprocessors, resistors, and circuit pathways that make your computer operate. This board also contains expansion slots, in which you insert expansion cards, extending the capabilities of the system.
This can depend on the printer, but normally the printed side is face up, you insert it printed side that you can see.
easily
You insert blank CDs in your computer and burn your songs onto them and then insert the CDs into the other computer and import them.
i assume main switch in that case at the mains input.
a insert
True.
in the USB port
on the computer
open up the computer, should be a label on the insert
It will not play, but it does not break your computer