Downtempo (sometimes referred to as "chill out", "chill" or "downbeat") is a type of electronic music style similar to ambient music, but usually with a beat or groove. The tempo, as well as the drum patterns of each track can vary. Sometimes the beat can be restrained and/or simple.
Sometimes the beats are more complicated and more featured instead of being in the background, but even then they are usually less intense than other kinds of electronic music like trance and house. The tempo is often slower than that of traditional electronic dance music.
Often the name chill-out music is used to refer to songs demonstrative of the genre, but those names also refer to other styles of music, and downtempo encompasses a wider variety of styles than those terms alone would indicate.
Due to the relaxing and often sensual or romantic feel of most downtempo music, it is a popular form of background music in 'chill out rooms' of dance parties, and many alternative cafes.
Source: Wikipedia, YouTube