Answer: both are used to write computer programs on.
Most office PCs, in contrast, chiefly run ready-written packages - word processing, perhaps, or spreadsheets - rather than programs developed in- house.
But it wasn't always that way. The first office computers - Commodore PETs and the like - had little ready-written software available. Users had to write their own.
This was fine, as that was usually the intention anyway: a cheap computer was needed, rather than a popularly priced package.
But once such packages caught on, people usually bought office computers to run a particular application: switch on, load it up and go. Programming was neither necessary nor efficient. Off-the-shelf programs were normally perfectly adequate.
That balance is now changing again - and sales of programming tools are booming.
Microsoft, whose UK programming language sales grew at only 3 per cent per annum in recent years, saw them shoot up by 59 per cent last year, according to Andrew King, the programming languages product manager.
The boom, it seems, has come about with the coincidence of a number of trends. The first is the growing popularity of Microsoft's mouse-driven 'Windows' operating software. Its promise of being relatively painless and hassle-free is introducing more and more users to the power of the PC.
As in the early Eighties, the computing platform has arrived before the programs - so users are again writing their own.
The company's 'Visual Basic' language, which carries out many of the chores of manipulating windows and responding to mouse movements, neatly fills this gap.
Another reason for the increased sales is that as PCs become more powerful, they can take over the operation of programs that a business formerly had to run on mainframe or minicomputers. Microsoft's Cobol and Fortran languages are selling well as a result.
The trend is confirmed by Iain Rangeley, director of Grey Matter Ltd, a leading UK language distributor.
Engineering and scientific programs, he says, are particularly suitable for this 'down-sizing'. It's essentially just a question of translating from one version of Fortran to another, and the pounds 2,000 PC of the Nineties can replace the pounds 30,000 minicomputer of the Eighties.
Nor is all down-sizing retrospective. Companies are increasingly writing programs for PCs in the first place. With PC price tags rapidly shrinking, more and more businesses are waking up to the opportunity.
City darling MicroFocus is an obvious beneficiary. Its leading product is a form of the mainframe language Cobol that runs on PCs. But ordinary people in ordinary businesses are writing programs, too.
Powerful versions of Basic, probably the world's most widely known computer language, are easily available and inexpensive. Microsoft's QuickBasic, for example, which offers a friendly front-end programming 'environment', retails for around pounds 59. Arch-rival Borland's offering, TurboPascal, is similarly priced.
Both offer the ability to compile programs, converting them into stand-alone pieces of code that will run on any PC, just as bought ones do.
The growing interest is reflected in the launch of Basic Magazine, aimed at serious Basic programmers. Editor Robert Schifreen accepts that Basic has an image problem, due to its use on home computers. But he points to a large number of programming professionals who earn their living developing applications with it.
There is also a substantial semi-professional interest. Basic is an easy way to get simple in-company utilities working quickly and cheaply, Mr Schifreen says.
Reflecting the need to write short and simple routines, Microsoft is launching WordBasic, a Basic-like 'macro' language specifically to help the general user to link applications. Users who thought that they couldn't write programs will suddenly find they can - and may be tempted to move on to more powerful languages.
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