An amateur radio satellite is an artificial satellite built and used by amateur radio operators for use in the Amateur-satellite service. These satellites use amateur radio frequency allocations to facilitate communication between amateur radio stations.
Many amateur-satellites receive an OSCAR designation, which is an acronym for Orbiting Satellite Carrying Amateur Radio. The designation is assigned by AMSAT, an organization which promotes the development and launch of Amateur-satellites. Because of the prevalence of this designation, Amateur-satellites are often referred to as OSCARs.
These satellites can be used for free by licensed amateur radio operators for voice (FM, SSB) and data communications (AX.25, packet radio, APRS). Currently, over 5 fully operational amateur-satellites in orbit act as repeaters, linear transponders or store and forward digital relays.
Throughout the years, amateur-satellites have helped make breakthroughs in the science of satellite communications. A few advancements include the launch of the first satellite voice transponder (OSCAR 3) and the development of highly advanced digital "store-and-forward" messaging transponder techniques.(Wikipedia)