A cable modem is the box that you connect between the cable wire and your network which may be just your computer.
It is not actually a modem, a modem coverts signals from one type to another, but the name stuck from when people used modems to connect their PC or network to a dialup connection. In that case telephone signals were converted to computer signals. The cable modem is working at same spot but differently. The cable modem connects your network to the computer company’s network.
When you get internet service through a cable provider they will supply you with a cable modem. Some cable modems have wireless access included and some do not which means if you want wireless you will need to buy a wireless router to use also.
The cable provider (ISP) will rent you the cable modem or you can buy your own cable modem. Some cable providers such as Comcast will tell sell you a modem or give you a list of approved modems. Others you will have to ask for a list if you want to buy one. If you buy your own cable modem you will have to give the cable company the MAC address (hexadecimal digits) for your service to work as that is how they know you are a customer.
Cable modems are continuously improving to support more traffic and faster. Currently they are defined as Docsis 3. Docsis is a way of rating cable modems.
I recently bought my own cable modem so I do not have to pay a monthly rental. However care of the box is my responsibility now.