In order to connect to the internet, or a home or office network, your computer must have either a modem for connecting to dial up internet, or an Ethernet card for connecting to a network or high speed internet.
Modems and Ethernet cards both send and receive communication so that you have access to some kind of network. Today, Ethernet cards are built onto most motherboards and do not require an add on card, however, some older computers may not have it built in.
If you currently have, or plan to use dial up internet then you will need to have a dial up modem.
If you currently have or plan to use high speed DSL, Cable Internet, FiOS, or a home or office Local Area Network then you will need an Ethernet port.