From Wikipedia:

"Orienteering is a adventure sport involving navigation with a
map and compass. The traditional form (sometimes referred to
as "Foot Orienteering" or "Foot-O") involves cross-country running, though
other forms have evolved. The competition is a timed race in
which individual participants use a special purpose map and a magnetic compass to navigate through diverse terrain (often wooded) and visit, in sequence, control points that are
indicated on the map. The course of control points is kept
secret from the competitors, until the start, when they are
provided with a detailed topographic map on which the course
is marked. Competitors start at staggered intervals, are
individually timed, and are expected to perform all navigation
skills on their own. Standings are determined first by
successful completion of the course, then by shortest time on course."

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A newcomers guide to orienteering on the British Orienteering Website

A beginners guide produced by MDOC is also available as a pdf download.