poplar is mostley used for prototype guitars. its grain is pretty flat and it usually has green stains in it. it is fairly cheap thats why they use it.
i would compare poplar is like a cheap grade alder, much like agathis is to mahogany

haha i just read that link... its completely different than what i thought.. i might be thinking basswood with the green stains.. but that site also is wrong about it being hard to find.. its at home depot.. unless they ment in the 60's. Ive always reguarded poplar as a prototype wood