The final stage in the development of pure sailing vessels
used as cargo ships. Technically, "windjammer" means the same as "topsail schooner" but the term is used for other ships.
Imagine a wooden or even a steel-hulled vessel, hundreds of feet long,
with as many as seven masts, and with more and larger sails than
you could imagine fitting in the available space, and you will have a good
picture of a late nineteenth-century windjammer.
The development of sailing craft had already been towards faster and
larger ships. The state of the art as of the mid-nineteenth
century was the two-masted schooner, and its fabled sub-type, the clipper. A fast clipper could get a small cargo from point A to point B faster than any other ship, if the winds were blowing right.
In the second half of the nineteenth century, steam was all the rage
-- as evidenced by Phileas Fogg's transatlantic trip on the General
Grant, steam-powered vessels were less prone to the vagaries of the
weather, and required fewer men to handle, than their sail-only counterparts.
Shipping lines saw that steam-powered vessels were simply more profitable
than sailing vessels, and they began using steam-powered vessels more and
more. Steamboat crews began to deride the sailing vessels that remained
as "windjammers".
There was still one niche that sail had not been pushed out of --
speed. As the nineteenth century progressed, people began to manufacture
larger and larger schooners with more and more masts. The final
development came when people began rigging these schooners with oversized
sails -- pushing mast and sailcloth technology as far as it could go.
At some point the once-derogatory term began to be used by ship designers
and sailing crews themselves.
These ships were very fast, but their days were numbered. The
introduction of the diesel motor, as well as the screw propellor, took
away sailing ships' last advantage. The persistence of sailing craft
hauling cargo became a matter of tenacity. Although some windjammers
were used to haul cargo (and even troops) as late as World War II, they
were on their way out. Windjammers were especially ripe targets for
U-boats.
These days, a "windjammer" is any large sailing vessel (approaching
100 feet (~30m) or more) used for pleasure cruising. It is occasionally
used to mean any vessel rigged with oversized sails. The tall
ships kept by various navies for training purposes, and as goodwill ambassadors, are frequently windjammers.