The ZX81 was the successor to the ZX80 and was produced by Sinclair Research (founded by Sir Clive Sinclair) from around 1981 to 1984. It produced a black and white text display on a domestic TV, had a touch sensitive keyboard and contained 1KB of RAM (yes, one kilobyte or 1024 8-bit bytes of memory.)
Additional memory was available as plug-in expansion packs. Data was loaded using a domestic tape player/recorder via audio in/out jack plugs.
There is a view that the ZX81 helped start the Home Computer market in the UK. There was a great deal of interest in the machine especially considering the relatively affordable price (about equal to the 200 UKPounds price point in 2009)