Letters have been around since the invention of paper. They are used to pass on information or opinions when you cannot be there in person.
One of the first proper postal services was the pony express. In March 1860, William H. Russell, an American transportation pioneer, advertised in newspapers "Wanted: Young, skinny, wiry fellows not over 18. Must be expert pony riders willing to risk death daily. Orphans preferred."
Russell had failed repeatedly to get backing from the Senate Post Office and Post Roads Committee for an express route to carry mail between St. Joseph, Missouri; the westernmost point reached by the railroad and telegraph; and California. St. Joseph was directly 2,000 miles to California and except for a few forts and settlements the route beyond St. Joseph was a vast, unknown land, inhabited by Native Americans.
Many people believed transportation across this area was impossible because of the extreme conditions. Russell, however, thought the route was feasible and was ready to organize his own express to prove it.
As a first step, Russell and his two partners built relay stations.
Starting on April 3, 1860, the Pony Express ran through parts of Missouri, Kansas, Nebraska, Colorado, Wyoming, Utah, Nevada, and California. On an average day, a rider covered 75 to 100 miles. He changed horses at relay stations, set about 10 or 15 miles apart, transferring himself and his mochila (a saddle cover with four pockets or cantinas for mail) to the new mount, all in one leap.
The first mail by Pony Express via the central route from St. Joseph to Sacramento took 10 1/2 days, cutting the Overland Stage time via the southern route by more than half. The fastest delivery was in March 1861, when President Abraham Lincoln's inaugural address was carried in 7 days and 17 hours.
From April 1860 through June 1861, the Pony Express operated as a private enterprise. From July 1, 1861, it operated under contract as a mail route until October 24, 1861, when the transcontinental telegraph line was completed.
But now nearly each country has its own postal service and Fedex and Air Mail services transport letters globerly.