Rich black is a combination of 100% black ink and an additional mixture of cyan magenta and yellow ink.  By default 100% black in almost all print drivers and RiP's is set to overprint.  What this means is that if an image is behind a black title block, the colors of that image still print on the sheet and then black is printed over top. A rich black helps prevent other colors that are behind the black from showing through as well as letting you make the black seem darker.  You can also change the temerature of a rich black based on what build you use.

Our recomended rich black build is:
C: 40%
M: 30%
Y: 30%
K: 100%

This example image shows how a 100% black box looks when printed over top of other colors.  The area in the middle is a "rich" black and shows how the black is darker than just pure black ink and can prevent the other colors from showing through.

Where you do not want to use a rich black is on small black text.  Because the 4 primary print colors have to be registered or alligned when printing, a rich black build of fine type or text will show the tiniest misregistration on the print making the text not appear crisp and sharp.