http://en.wikipedia.org/wiki/Clothing_terminology
http://www.apparelsearch.com/glossary.htm
http://septicscompanion.com/showcat.php?cat=clothing
http://en.wikipedia.org/wiki/History_of_clothing_and_textiles
http://www.elizabethancostume.net/
you can always type in the garment name in google and punch in images and see what pops up. Check wikipedia etc...
Do be careful to check whether the garment name was actually in use at the time of your setting or a modenr invention to describe old clothing. Some terminology changes over time like stays to corset between the 18th and 19th centuries.
I'll spare sending you to web sites and give you some good hard books to pluck from Amazon or your library,
20,000 Years of Fashion by Francois Boucher.
400 Years of Fashion by Avril Hart (from the V&A)
http://www.amazon.com/400-Years-Fashion-Natalie-Rothstein/dp/1851773010/ref=cm_lmf_tit_9_rsrssi1
These are 2 good survey books with a broad overview of western fashion. For other parts of the world or time periods, you'll need more specific books.
My problem is that I often know the look, but not the name. So it took forever to figure out the term jodhpurs when I wanted a person in those pants that flare above the knee but were tighter from the knee to the ankle. Google doesn't handle that kind of search well. It's kind of like when a person asks how to spell something you tell them to look it up in the dictionary. The picture books and such will be most helpful, thanks.