The SG-1000 is Sega's first attempt at a video game console. Despite being a rather powerful machine for its day, it was a victim of horrible timing: it came out the exact same day as the Famicom, which almost immediately took over the console market. Because of this, the SG-1000 had a very limited release (only Japan and New Zealand), and its game library remained relatively small. The main problem with the SG-1000 was its controller: it was clunky and unresponsive, and replacing it was a huge headache, since it was hardwired into the console.
Since this system bombed so hard, Sega was quick to work in a successor, the Mark III aka Master System.