It seems that valance is the general term for a decorative accent that covers the curtain rod and hardware or the top of a window in general. A pelmet is a stiff valance that is typically rectangular, and usually has curtains hanging down from it. A lambrequin is a valance whose sides come further down the window; it is often — but not always — a stand-alone feature with no curtains or blinds beneath it.
Most of these window treatments look more formal and "finished" because of the inclusion of valances, but we can also see that it's an opportunity for whimsy and flamboyance that can add drama and fun to a room.
Which is your favorite?
1 "DIY" valances by Jennifer Eisenstadt in Domino, via materialgirlsblog
2 Clean white trim on a green pelmet via Dwellers Without Decorators
3 A black-trimmed pelmet make the camel curtains feel very Chanel-chic, and echo the pillow trim in this room designed by Amy and Todd Hase and photographed by Roger Davies for the September 2010 Elle Decor
4 A continuous valance with orange trim in a bedroom designed by Victoria Thompson from the Feb/Mar 2010 Lonny
5 The Reluctant Blogger made this box pleat valance herself and has a nice DIY guide here
6 Now we're getting away from simple box pelmets into more complex designs, like these designed by Scott Currie and photographed by Roger Davies for the July 2009 Elle Decor
7 Jonathan Adler put a small fabric valance at the top of these shades, via Elle Decor
8 Mary McDonald put a whimsical valance around her curtain walls in her guest house, photographed by Miguel Flores-Vianna for Domino, via Bride's
9 A striped lambrequin in a home designed by David Kleinberg and photographed by Eric Piasecki for the January 2007 House Beautiful
10 These lambrequins provide maximum drama in this room designed by Miles Redd and photographed by Miguel Flores-Vianna for the December 2009 Elle Decor
Images: as linked above