The Ethernet card is by no means an essential part of a home PC. Unless you wish to do some home networking or have a broadband (broad bandwidth) internet connection, such as Cable, or ADSL. With cable you will have a cable modem that has a cable (just like the cable for your TV at home) the plugs into it, and It has a twisted pair Ethernet wire that comes out of it and hooks into your Ethernet card. I am not going to go into the differences between a twisted pair, and straight through CAT 5 cables, except for the fact that if you wish to hook up two computers with CAT 5 cable you must either have a hub and two straight through cables, or one twisted pair cable. Most Ethernet cards are capable of transferring 100Mbps (Megabytes per second). This card shown here can do 200Mbps under certain conditions. The big difference between the physical appearance of this card and a modem is that the size of the port on it is obviously bigger than a standard phone jack. After all it is broadband, not a phone line.
On the fun side Ethernet cards are great for setting up a LAN (local area network) and sharing files, printers, and playing games with your friends. If you don't mind toting your PC around to peoples' houses you will have tons of fun playing LAN games against your friends. The possibilities are limitless when you set up a home network. Just make sure that you have the fastest computer be the server.
I gave the 10/100 Etherfast LAN card a 98% for greatness. It is only about $15 and it does the job just as good as the $50 models. This is a great card, very durable too, I have an extra card that I let my friends borrow when we have LAN parties, and It has been pulled in and out of PCs dozens of times and it still works just fine. This is the way to go for your Ethernet card!