As you probably already know, musicians categorize the human voice according to range using terms like bass, baritone, tenor, countertenor, contralto, and soprano. According to the Baker's Student Encyclopedia of Music available on eNotes, the tenor has a typical range of "two octaves, from the C below middle C to the C above (c0-c2)" and a baritone is "the medium-range male voice, lower than the tenor and higher than the bass, with a compass from A to about f1." According to European classical tradition, voice types can be classified even further than just range, using the "weight" of a voice.
Think about different tenors you may have heard. Some of them have a darker, heartier, earthier, heaviness to their tone. Others seem lighter, fluid, and soaring. These two tenors may have the same range, but they have different vocal weights. Wikipedia calls vocal weight "the perceived "lightness" or "heaviness" of a singing voice." The article goes on to state:
Lighter voices are often associated with the term lyric and are usually brighter and more agile; heavier voices are often associated with the term dramatic and are usually powerful, rich, and darker.
Lyric voices are those with a lighter vocal weight. Keep in mind that this doesn't neccesarily mean that lyric sopranos or lyric baritones can sing higher that others in their vocal range, but they sound "lighter." Lyric voices may not have the power of their dramatic counterparts, but when it comes to long, fast runs of notes in their range, they make it sound angelic and effortless.
The "lyrics" of a song are the words that are sung to the melody or tune. Singing voices are described in general as being soprano (the high female voice), alto (the low female voice), tenor (the high male voice), or bass (the low male voice). Within each of these four classifications, however, there are a wide range of voice qualities - some voices are described as being "dark," others might be described as "full," others as "clear" or "throaty." These terms explain the particular characteristics of the sound of a particular vocalist. A "lyric tenor" has a "light" quality to his voice and is probably capable of singing clearly and musically the very highest pitches within the male vocal range. Luciano Pavarotti was a lyric tenor.