What is the difference between DOT brake fluids?
From Wikipedia:

Part of the difference is boiling points:


               Dry boiling point   Wet boiling point
DOT 3       205 °C (401 °F)     140 °C (284 °F)
DOT 4       230 °C (446 °F)     155 °C (311 °F)
DOT 5       260 °C (500 °F)     180 °C (356 °F)
DOT 5.1     270 °C (518 °F)     190 °C (374 °F)


3, 4, and 5.1 are glycol-based, and must be changed periodically, as it absorbs water. 5 is silicone-based and does not need to be replaced.