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A calculator is a small (often pocket-sized), usually inexpensive electronic device used to perform the basic operations of arithmetic. Modern calculators are more portable than most computers, though most PDAs are comparable in size to handheld calculators.
The calculator has its history in mechanical devices such as the abacus and slide rule. In the past, mechanical clerical aids such as abaci, comptometers, Napier's bones, books of mathematical tables, slide rules, or mechanical adding machines were used for numeric work. This semi-manual process of calculation was tedious and error-prone. The first digital mechanical calculator was invented in 1623 and the first commercially successful device was produced in 1820. The 19th and early 20th centuries saw improvements to the mechanical design, in parallel with analog computers; the first digital electronic calculators were created in the 1960s, with pocket-sized devices becoming available in the 1970s.
Modern calculators are electrically powered (usually by battery and/or solar cell) and vary from cheap, give-away, credit-card sized models to sturdy adding machine-like models with built-in printers. They first became popular in the late 1960s as decreasing size and cost of electronics made possible devices for calculations, avoiding the use of scarce and expensive computer resources. By the 1980s, calculator prices had reduced to a point where a basic calculator was affordable to most. By the 1990s they had become common in math classes in schools, with the idea that students could be freed from basic calculations and focus on the concepts.
Computer operating systems as far back as early Unix have included interactive calculator programs such as dc and hoc, and calculator functions are included in almost all PDA-type devices (save a few dedicated address book and dictionary devices).
In addition to general purpose calculators, there are those designed for specific markets; for example, there are scientific calculators which focus on operations slightly more complex than those specific to arithmetic – for instance, trigonometric and statistical calculations. Some calculators even have the ability to do computer algebra. Graphing calculators can be used to graph functions defined on the real line, or higher dimensional Euclidean space. They often serve other purposes, however.

(Taken from: Wikipedia.org Website: (http://en.wikipedia.org/wiki/Calculator))