Could someone explain what a Supergun is to me?

I hear about these things alot, but what are they?

A device thats allow you to play Jamma(arcade game)at home(TV) without a cabinet.

Theres a few topics on this, use the search function on the top.
or you can visit the website at
<a href="http://www.sgrepository.com/modules.php?name=gallery" target="_blank">supergun</a>

Thanks.