The ZX81 was designed to be a low-cost introduction to home computing for the general public. More than 1.5 million units were sold.
The ZX81 was equipped with 1KB memory, the operating system was driven by a Basic variant called Sinclair Basic. Programs or data can be stored on compact cassette's by connecting them to an audio output of the ZX81 (EAR).
The ZX81 had an Rf output to be connected to a TV and produces no sound.
Expansion modules existed like a 16KB memory expansion modules and a printer. Also many other third party expansion modules were developed, like better keyboard, printers and others.
Most commercial software like games or tools were written for use with a 16K expansion module.
The ZX81 was known as the Timex 1000 on the US market.