The Modern Day Sorcerer

    ... we conjure the spirits of the computer with our spells.

    A computational process is indeed much like a sorcerer's idea
    of a spirit. It cannot be seen or touched. It is not composed
    of matter at all. However, it is very real. It can perform
    intellectual work. It can answer questions. It can affect the
    world by disbursing money at a bank or by controlling a robot
    arm in a factory. The programs we use to conjure processes are
    like a sorcerer's spells. They are carefully composed from
    symbolic expressions in arcane and esoteric programming languages
    that prescribe the tasks we want our processes to perform.

    A computational process, in a correctly working computer, executes
    programs precisely and accurately. Thus, like the sorcerer's
    apprentice, novice programmers must learn to understand and to
    anticipate the consequences of their conjuring. Even small errors
    (usually called bugs or glitches) in programs can have complex and
    unanticipated consequences.

SICP, Chapter 1, lifted from the on-line version:

http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-9.html