At 12:26 Universal (Greenwich) time, or 5:26 a.m. Pacific time, the Sun will reach its highest declination in the sky (a celestial version of latitude). This is what astronomers call the summer solstice, and what Americans incorrectly call the first day of summer.
It means that at local noon (literally, halfway between sunrise and sunset), the Sun will be higher in the sky than it will be for the rest of the year, and every day until December 22 (the winter solstice) it will dip a bit lower. If you live in the southern hemisphere, than this means it's the shortest day of the year, and the Sun will get higher every day, yadda yadda. Just reverse what I said above, or stand on your head if that helps.
Wanna know when the solstices and equinoxes are for the next couple of decades?
High Hie thee to the United States Naval Observatory astronomical almanac. There's lots more fun stuff there that'll keep you amused, too.
Note added August 14, 2006: I was informed by a reader that it's "hie thee", not "high thee". I'm an astronomer, not a middle English grammarian, Jim!'