There's a pretty good writeup about it in Wiki.
http://en.wikipedia.org/wiki/Daylight_saving_timeQuote:
The practice has been both praised and criticized. Adding daylight to afternoons benefits retailing, sports, and other activities that exploit sunlight after working hours, but causes problems for farming, evening entertainment and other occupations tied to the sun. Its effect on health and crime is less clear. Although an early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity, modern heating and cooling usage patterns differ greatly, and research about how DST currently affects energy use is limited or contradictory.
DST clock shifts present other challenges. They complicate timekeeping, and can disrupt meetings, travel, billing, recordkeeping, medical devices, heavy equipment, and sleep patterns. Software can often adjust computer clocks automatically, but this can be limited and error-prone, particularly when DST protocols are changed.
Another interesting point, mentioned later, is that DST is not a new invention. It's just more rigidly defined now. Ancient cultures based their times of day on the position of the sun, so their time-judgements automatically adjusted themselves for differences in daylight hours.
I do have to say, given a choice, I'd stay on DST all year round, rather than do without it or turning the clocks back in the fall. Standard time is never a good idea. I like the extra sunlight hours later in the day, rather than early in the morning.