I first heard a Baroque Flute played by Stephen Schultz in a recital he was giving at the San Francisco Conservatory of music. I thought, "what a nice sound, and that French music is really beautiful and different". A little later, Stephen lent me a real dog of a flute to try out over the summer, but it couldn't have been too bad (I should try it again sometime) as I didn't end up wanting to use it for firewood, but could actually play some of the Bach triosonatas on it.
The instrument differs from the modern counterpart in that the bore is conical; much narrower at the footjoint that at the headjoint. The finger holes are also much smaller, as is the blowhole, producing a mellower sound than the modern metal instrument. Of course, the flute is made out of wood. Usually ebony or boxwood, but sometimes other woods like snakewood, rosewood and maple.
Playing on it really interesting, as the different keys have different qualities on the flute. This is because some of the notes, particularly in the low register, are produced by fork-fingerings making the sound weaker and softer. The difference between D Major and E-flat Major is pretty astounding.
I also enjoy playing later flutes with more keys on them. The design is basically the same, but instead of just one key on the footjoint, there are keys spaced all over the three lower pieces, strategically placed to let one play strongly those notes that would be weak on the one-key instrument.
to make this into a (hopefully) useful internet source.