This specific category of computers existed primarily in the 1980s. Manufacturers included Casio, Hewlett-Packard, Sharp, Tandy/Radio Shack (Selling Casio and Sharp models under their own TRS line) and many more.
The programming language was usually BASIC, but some devices offered alternatives. For example the Casio PB-2000 could be programmed in Assembly, BASIC, C, and Lisp. An important feature of pocket computers was that all programming languages were available for the device itself, not downloaded from a cross-compiler on a larger computer.
Though not identical, in principle personal digital assistants, handheld PCs, and programmable calculators serve many of the same functions as the old pocket computers, generally with significantly more computing power in a package the same size or smaller. The main distinction is that more modern designs (with the exception of programmable calculators) usually do not have included programming ability and are usually set up to act as clients of a larger system rather than as self-contained environments of their own, whereas the early pocket computers had their own data storage and input/output facilities such as printers and tape drives.