It sounds like you want to use your computer monitor as a TV
If you just want to watch cable from your cable box on your monitor then you wont need a tv tuner.
Instead all you have to do is directly hook up your cable box to your monitor, like you would do with your TV. But first you'll need to check whether your cable box uses HDMI or component video (the red green and blue cables)
If your cable box has HDMI output then directly connection it to the HDMI input on your monitor, like you would do with your tv.
If it uses component video (the red green and blue cables) instead of HDMI then you may be out of luck, unless you can find a component to HDMI converter.
If however you want to use your actual computer running windows as a replacement for your cable box you will need a tvtuner
What a tvtuner does is allows you to watch cable on your computer instead of using a cable box. A cable box is far superior though because tvtunes have a lot of limitations, for example you cant watch HD content. It's suspected that cable companies implant limitations on tvtunes so that people will have to rent/buy the cable company's cable boxes. The FCC is currently investigating this for anti-competitive practices.
So if your looking to get a TV tuner I'd commend just getting a cable box. It's up to you though.