The terms HDTV 'receiver', 'tuner', 'box', 'STB - set-top-box' all refer to external set top boxes that are used to receive HDTV, and provide video and audio output.
Some STB's are for a specific provider, like a cableco (Comcast, TWC, etc.) or DBS provider (DirecTV or Dish), while others are for over-the-air Digital TV (DTV) reception.
For provider specific STB's you need to be connected to their system. With a cableco that's done using a coax cable they provide. With DBS, the coax is connected to a dish that is aimed to receive their satellites.
For over-the-air DTV, the STB is connected to an antenna, which receives the local broadcast stations in your area. Generally speaking, DTV reception is limited to 60 miles from where the broadcast stations are located, although exceptions are common either way; closer stations may be troublesome and more distant stations may be available.
To learn where the stations are in your area, go to TV Fool.com, and punch in your address and zip code.