An Up Cut bit is very efficient in evacuating chips up and out of the cut.
It will leave a very clean finish at the bottom of the work piece, but will leave a rougher surface on the top of the surface (or the side that the bit enters the work piece).
A down cut bit does just the opposite. It is best to use a down cut bit for through cuts, as a down cut bit pushes the chips down into the cut.
Down cut bits will leave a very clean cut on the top of the work piece, but may leave a rougher finish on the bottom.
Down cut bits are also great for cutting shallow dados, rabbets, and visible slots because of the clean finish it gives on the edge of the hole or groove.
When cutting dados or grooves with a down cut bit, be sure to slow the feed rate to allow more time for chip removal