Yes, you can watch television with an antenna, especially if you have a digital TV converter box that allows your analog TV to receive the digital signals that all U.S. full-power broadcast television stations now use. The advantages of switching from analog to digital broadcasting include the availability of parts of the broadcast spectrum for emergency usage by the police, fire or rescue operations; enhanced audio and better picture quality; and the possibility for interactive video and data services. If the antenna on your roof or indoors, which you used to receive analog broadcasts, gave you decent or wonderful quality, you should be able to continue to use it for digital broadcast signals.
You'll want to make sure that any new antennas you get are able to receive both UHF (ultra high frequency) and VHF (very high frequency) channels, and that the reception is good. Usually, rabbit ear antennas (two antennas sticking up like rabbit ears) are used for receiving signals from UHF stations, while a wire loop or bowtie antenna is good for VHF stations. If you live pretty close to your nearest broadcasting station and the terrain between you and it is fairly flat, you may be able to get away with an indoor antenna. If you're experiencing trouble with your antenna receiving the digital TV signals, it could be that you need to adjust the antenna to be in line with the broadcasting tower, or it could be due to mountains or tall buildings interfering with reception. You may need an outdoor antenna in this case.
Don't bother buying anything other than a digital TV converter box, until you check whether your existing antenna already picks up all the stations you want by performing a channel scan. Press the set-up or menu button on your converter box and use the direction arrow buttons to scroll to the option that lets you search for digital broadcast channels available where you live. Every now and then, check if more digital channels are added.