A lot like a network adapter, an Ethernet card supports the standard for a high speed wired connection. They are also known as network interface cards (NICs).
These days Ethernet cards use PCI standards and are often installed inside Pcs. They are also available for laptops in the shape of card like devices that slide into slots at the side of a laptop (these are usually PCMCIA Ethernet cards or Pc cards, however bear in mind not all of them support Ethernet).
USB Ethernet devices are also available, these are ideal for units without a PCMCIA slot and for computers which are often connected to gaming consoles.
The protocol standard they support dictates the speeds they run at. Most new ones can manage speeds of up to 1gbps. Also, whereas they cannot function as standalone supports for wireless networking, they can be used with broadband routers to allow connections via cables and communication to Wi-Fi devices through the router.