Standard Definition TV (SDTV) - SDTV is the basic level of quality display and resolution for both analog and digital. Transmission of SDTV may be in either the traditional (4:3) or widescreen (16:9) format and consists of 480 interlaced lines of resolution.
Enhanced Definition TV (EDTV) - EDTV is a step up from analog television and Standard Definition digital television. EDTV comes in widescreen (16:9) or traditional (4:3) format and provides 480 progressively scanned lines of resolution. This is approximately twice as much resolution as SDTV and is the format most often found on DVDs.
High Definition TV (HDTV) -
HDTV in widescreen format (16:9) provides the highest resolution and picture quality of all digital broadcast formats. It consists of either 1080 interlaced or 720 progressively scanned lines of resolution and includes the ability to transmit digital surround sound.
Source:
The Digital TV Transition: FAQs - Consumer Corner - General Questions