Sinclair was the brain child of Clive Sinclair, better know as Sir Clive these days (Knighted 1983) and the ZX80 and ZX81 where the first home computers to be sold by the Sinclair Company. Note: Sinclair did make a previous machine called the MK14 but this was nothing more than a glorified calculator.
The ZX80 was the first to hit the market in 1979 and it could be said is the machine which founded the home market in the UK. Sold in either kit form (Which you put together yourself, about £75) or ready built @ £99.95.
To keep the price down on the ZX80 the designers used a low cost touch sensitive keyboard which was a piece of plastic with touch sensitive pads underneath. This made it very unsensitive!
The ZX80 used a 3.25Mhz processor with 1 kilobyte of RAM (Expandable to 16k with a RAM Pack) and had a limited BASIC which was on board. The machine connected to the TV via an RF out socket on the back but was limited to monochrome only with no sound. Other connectors on the machine were for the RAM pack (Sinclairs Z80 bus) and a audio in/out connector for saving/loading programs to cassette.
The machine had no graphical capabilities using text only at 24 lines by 32 characters.
The ZX80 sold around 100,000 units and finding one today is difficult, especially one that works. Finding one that is still in it original box is even harder and prices have been known to reach the £350 mark on ebay.
The ZX81 was the next progression in the Sinclair camp and although technically similar to the ZX80 the ZX81 proved a bigger success.
The ZX81 used the same processor and had the same RAM as the ZX80, but came with 8K ROM with memory being upgradeable to 64kbytes. Again the machine supported no graphics and no sound and had an equally unusable/unresponsive touch sensitive keyboard. However as Sinclair sold this unit in the high streets, noticeably WH Smiths, the machine sold over 1 million units between its release in 1981 to 1982 costing £69.95.
Software for these machines was extremely limited as most programs came in booklet form which had to be typed in, however once typed in they could be saved to cassette using the Audio in/out leads. This meant you could load them in again at a later date.
Some off the shelf software did emerge such as Asteroids and Breakout clones.
Believe it or not you could buy a printer for these machines, which Sinclair themselves made. This was a thermal printer and used a silver/grey looking paper which was headed by the printer to form characters, a bit like a fax machine. Although I have one of these I have yet to find anyone who has any paper for it!