What are buses when talking about network cards?

It's talking about where you plug the NIC (Network Interface Card) into, the slot, on a motherboard. This is an internal and external bus. The external bus is also called the expansion bus. However, when referring to a card to plug into the bus, it's the external bus.
There are different types of cards and different types of slots. The slots are color coded based on their type. This helps explain the different types:
http://www.ehow.com/info_12183745_color-slot-indication-motherboards.html

The paths taken by data moving with a computer are called "buses". Multiple side-by-side paths force data to move in parallel, and not in series (one after another).