On October 4th, 1957 at 19:28:34 hours Greenwich Mean Time, Sputnik-1 was launched into space from the Baikonur Cosmodrome.
The satellite orbited the Earth for three months and emitting radio signals which were monitored by amateur radio operators throughout the world. The signals continued for 22 days until the transmitter batteries ran out on October 26th, 1957.

Before finally burning up during reentry on January 4th, 1958, the satellite traveled a total of about 60 million km (37.28 million mi) and completed 1,440 orbits around the Earth. Sputnik-1 also helped to identify the density of the atmosphere’s upper layer, provided data on radio-signal distribution in the ionosphere, and allowed for the first opportunity for meteoroid detection.

 Design and Construction:

Initially, the Soviet plan for an satellite (known as Object D) was planned to be completed in 1957–58, and called for the creation of a spacecraft that would have a mass of 1,000 – 1,400 kg (2,200 – 3,100 lb) and would carry 200 – 300 kg (440 – 660 lb) of scientific instruments. In terms of tasks, the mission would seek to measure the density of the atmosphere and its ion composition, solar wind, the Earth’s magnetic field, and cosmic rays (largely for the sake of future missions). A system of ground stations was also called for in order to collect data transmitted from the satellite, as well as observe its orbit and transmit commands.

By the end of 1956, it had become clear that the specifications called for were too ambitious to be accomplished within the established time frame. Fearing the US would launch a satellite before the USSR, Korolev and the OKB-1 suggested that a simpler, lighter satellite could be launched in April-May 1957, before the IGY began.

This satellite would weight about 100 kg (220 lbs) and would forgo heavy scientific instruments in favor of a simple radio transmitter. On February 15th, 1957, the Council of Ministers of the USSR approved this simple satellite, designated “Prosteyshiy Sputnik” – Russian for “Simplest Satellite” – (aka. Object PS), and made arrangements to launch two versions (PS-1 and PS-2) using R-7 rockets.

 Sources:
NASA History – Sputnik
NASA: Solar System Exploration Guide – Sputnik
Russianspaceweb – Sputnik 1
Windows2universe – Sputnik