Breakbeats were historically made from the "breaks" in drum lines from older songs... these breaks would typically last only about a measure, and were used by drummers to add a more energetic, syncopated transition between patterns or themes in a song, often just before or after the bridge or chorus. Somebody discovered that looping these breaks, instead of regular drum patterns, made for a more energetic, syncopated beat... but nowadays the style breakbeat refers to any beat which sounds like those first breaks, and can be made from scratch.