Before French horns (also called simply "horns") were used to make music, they were used to send signals. Hunters used horns to communicate with one another in fields and forests. One signal could mean "Come here," while another might mean "Let's call it a day."
The circular shape of the horn made them convenient. Hunters could "wear" their horn by putting their arm through its brass hoop. Horns became musical instruments in the 1600s, shortly after opera was invented. Horns were incorporated onstage, as a special effect in hunting scenes. This was so successful that horns soon became a regular part of the orchestral accompaniment for opera. Over time, they were used in other kinds of orchestra music, too.
Horns had one problem, though. A single length of tubing can't produce every desired note. Horn players had to carry several different horns with them, or bring "crooks," extensions that fit in the horn to make it play in different keys.
In the 1700s, horn players learned that by putting a hand inside the bell, they could create the notes that weren't available using the open horn. For some notes, they placed their hand all the way in the bell; for others, they placed their hand partly in the bell. The notes that could be played open were still played open. Because the notes were played with different hand positions, however, they didn't have the same tone quality. This was solved in the early 1800s, when valves were invented. Then French horns (and trumpets and tubas) could play every note of every scale, just like woodwinds, strings, and keyboard instruments.
The modern horn is a double horn, similar in concept to trombones with attachments. A fourth valve was added along with extra tubing for the first three valves. The horn player can easily shift back and forth between a horn pitched in F and one pitched in high Bb. This gives the player many more fingering options, and allows the player to switch back and forth as needed. The Bb side is primarily used for the higher register, while the F side is used for middle and low registers.
Information found at http://www.sbgmusic.com/html/teacher/reference/instruments/brass.html
