The story of the Louisiana Purchase starts with the Mississippi River. Back in the early years of the United States, travel by boat was generally the most effecient way to transport large quantities. Most major rivers of the United States drained into the Mississippi, which in turn drained into the Gulf of Mexico. This allowed materials to easily be transported from the Atlantic Ocean to the river and vice versa. The Mississippi was a very important river (and still is).
France owned the Louisiana Territory, and thus the Mississippi and New Orleans (which was where the Mississippi met with the Gulf of Mexico). This made things more difficult for American merchants and sailors than they would be otherwise. So, partially in the hope of promoting westward expansion, President Thomas Jefferson requested that two ambassadors travel to France in an attempt to purchase New Orleans. The two ambassadors, James Monroe and Robert Livingston, were authorized to spend ten million dollars in the purchase.
When Monroe and Livingston arrived in France, they were met with a very surprising offer. Napoleon Bonaparte had offered to sell the entire Louisiana Territory for only fifteen million dollars. Napoleon's reasons were three-fold: A slave revolt in Haiti had caused France to lose their stepping stone to America, money was needed for the French army, and nobody really knew exactly how large the territory was. Madison and Livingston were not authorized to pay that much, but knew that the time required to go back to the US, get the approval of Congress, and return to France would be much too great. Some other country might have already made the purchase. So, the territory was purchased, and the ambassadors returned to the United States.
Acquiring the territory alarmed Jefferson, however. Being a Democratic-Republican, Jefferson advocated a strict interpretation of the Constitution, which did not explicitly give the country powers to aquire new land. After debating with himself (and surely others), Jefferson decided to exercise the elastic clause and aquire the territory.
The purchase is sometimes referred to as the best thing that Jefferson did during his administration because of all the benefits. The territory had a vast amount of resources, including some of the best farmland in the world. Patriotism was increased because the public could say, "We went from thirteen measly colonies to this in only a couple decades!" Also, the power of the federalist (the opposing political power of the Democratic-Republicans) was reduced, due to their opposition to the purchase.
The Louisiana Territory was described as all the land west of the Mississipi river that had rivers flowing eastward.
The thirteen states formed from the territory are: Arkansas, Colorado, Iowa, Kansas, Louisiana, Minnesota, Missouri, Montana, Nebraska, North Dakota, Oklahoma, South Dakota, and Wyoming.