ahhh, reliability and validity

Reliability and validity are concepts that many students (and researchers!) have a hard time keeping straight.

Validity: the extent to which an empirical measure adequately reflects the real meaning of the concept under consideration. (how closely do you come to hitting the bull's-eye (presumably the concept you actually want to measure?)

Reliability: whether a particular technique, when applied repeatedly to the same object, will yield the same results each time.

With that in mind, some pictures:

The "hits" are far from the bull's-eye (low validity);
and all over the place (low reliability). 
Hits are closer to the bull's-eye (higher validity); but
the hits are also all over (low reliability).
Hits are FAR from the bull's-eye (low validity); but they are
clustered in the same area-- the same answers are
received with each test (high reliability).
(more common with survey research (of course we strive
to hit the bull's-eye!))
Hits are clustered in the bull's-eye-- high validity and high reliability.