DST: Daylight Saving Time - See [1 ] for info on the history, rationale, laws, and dates relating to Daylight Saving Time.
Not all locations observe DST. For instance, reading http://www.infoplease.com/spot/daylight1.html , one reads (at least during early Feb 2007) that most of 2 US states, as well as 4 US territories do not observe DST. Note that this page also discusses DST in other countries as well.
I used to hear, back in the olden days, people referring to Indiana as being the only place that didn't observe DST. Since 2005, that state does observe DST - but the state is split across two time zones, so software will still have problems dealing with timezones there.