Among the 118218 stars measured by Hipparcos, 11597 were found to be (possibly) variable. Of these more than 8000 are new. If you want to find some background to understanding what variable stars are, and why they are important in astronomy, take a look at the homepage of the American Association of Variable Star Observers.
Variables are broadly classified as follows (the star in brackets is a typical example for that class and is linked to the Hipparcos light curve for that star):
Two Annexes of the Hipparcos and Tycho Catalogues are dedicated specifically to variable stars.