Sacagawea was Shoshone. When she was young, she was taken by Hidatsa, another Native American tribe. She then became the property of a French man named Toussaint Charbonneau. He won her as the result of a bet and later married her. When Lewis and Clark met her, in 1804, she was 16. She was also about to become a mother.
Meriwether Lewis and William Clark hired Charbonneau and Sacagawea to guide them westward. She spoke two Native American languages: Shoshone and Hidatsa. He spoke Hidatsa and French. She had family out west. They both also knew a lot about the West and what to expect there.
Just before the expedition began, Sacagawea gave birth to a son. She carried the baby on her back during the entire journey--both directions. When the family returned home, in 1806, the baby was 2.
Sacagawea proved very useful to Lewis and Clark on the journey. Her Shoshone relatives provided them with supplies and safe passage through their lands. She saved several notebooks from a canoe that had overturned. She also served as a calming influence on other Native Americans, who saw the presence of a woman in the expedition as proof that it was not a war party.
When the two leaders split up on the way back east, Sacagawea went with Lewis. When they all returned to the Dakota Territory, she and her husband stayed behind while Lewis and Clark went back to St. Louis.
Sacagawea is thought to have died of fever in 1812, although some accounts say she lived to be 100.