You can find out what channels are available in your town by going to AntennaWeb.org. They have a useful online tool that will tell you what type of antenna you need to pick up specific HD channels.
Remember back in the day you had to adjust the rabbit ears to pick up channels? ATSC DIgital Tuners and antennas function on the same basic principles. The difference is the channels are crystal clear HD digital now. No more analog (NTSC) tuners and no more snow. But just because the signal is transmitted in a digital format that does not guaranty perfect signal reception. I have noticed that during high winds and rain the signal tends to drop out occasionally. With digital signals the signal distortion does not come in the form of snow. The signal simply drops out altogether and most tuners will have a default screen that pops up saying "weak or no signal".
If you have an HDTV but don't have an ATSC Tuner built in you can usually find DVD players that have ATSC Tuners built in. The signal flow will look something like this. The antenna will connect to the DVD players Antenna Input, which will be an "F" type connector. This cable that plugs into the DVD player will come with the antenna and be permanently attached to the antenna at the other end in most cases. If not you'll need a coaxial cable. Next you'll need to attach the output of the DVD player to your HDTV input with a COmponent cable or an HDMI. Remember audio can be embedded in an HDMI signal but not with a Component signal. If you decide to use a component cable you'll need to also make sure you use an Audio Cable to connect your DVD players audio output to your HDTV's audio input.