Computers normally have some form of expansion card containing the network interface controllers that are plugged into one of a number of different types of computer bus. Most desktop computers to date had a network card that fitted into a PCI slot. PCI stands for Peripheral Component Interconnect and was originally developed my Intel, but is now a standard used by almost all manufacturers that produce peripheral devices for connectivity with a computer.
As Ethernet has been the dominant Data Link Layer standard for Local Area Networks for some time, Ethernet Network Interface Cards have been produced in their millions to allow computers the connectivity through hubs, switches and routers with other computers. These network cards cost as little as a few pounds or dollars and allow computers to communicate across an internetwork and be identified by means of a unique physical address known as a MAC Address. A MAC Address is a 48 bit binary number, normally seen written in hexadecimal, and each Network Interface Card comes with a unique MAC Address.
Most people refer to such a network device as a NIC, Ethernet Card or Network Adapter and they will have an RJ45 female connector, into which a male connector on an Ethernet Patch Cable is connected. The patch cable has 4 pairs of copper wires, only 2 of which are used for normal Ethernet operation at 10 or 100 Mbps. Gigabit Ethernet uses all 4 pairs to achieve the higher data rate.
Any Network Adapter will require drivers, which are applications used by the computers to communicate with, and control the actions of the Adapter. Most modern computers will automatically detect a new device and apply the correct drivers, or prompt for the location of the drivers which may be on a disk supplied with the card in the case of a PCI Network Card.
When connecting a computer to a wireless network, a wireless network card will be required which will normally be a PCI card or more commonly now, a USB (Universal Serial Bus) adapter. The Wireless PCI card or USB wireless adapter will have an aerial or antenna designed to operate over the frequency range of the desired wireless network which will normally be the 2.4Ghz band for the ieee 802.11b and 802.11g standards and the 5Ghz band for the ieee 802.11a wireless standard. Although all wireless network interface cards must have an antenna, and normally do have a built in antenna, there are some models that allow the connection of an external antenna should a more appropriate antenna be required. This may be the case in certain circumstances where the internal antenna is not of sufficient quality or does not have the required gain to reproduce a good quality signal for demodulation and decoding.
Wireless networks are very popular because they do not require any cables to connect to the desired network and also, the USB adapter actually draws its power directly from the computer to which it is attached without the need for a separate connection. It is worth bearing in mind that wireless networks are subject to interference from other wireless networks and also from other devices also authorised to use the same frequency bands.
Although a lot of modern computers actually have the NIC built into the motherboard, there is still a need for network cards that plug into a computer as there are so many computers still functioning on networks that do not have motherboards with that capability.
There are many, many manufacturers of these peripheral devices and it is difficult to recommend one over another, except to say 'stick with well known brands'. The easiest way to purchase a cheap network interface card or wireless network adapter is certainly over the Internet from a specialist online retailer.