Sun sign astrologers take this basic twelve-fold division and relate all the current movements of all the planets to each other, using traditional rules to divine meanings for each sign separately. Paradoxically, because the Moon has the fastest apparent movement of all the heavenly bodies, it is often used as the main indicator of daily trends for sun sign astrology forecasts.
Despite its vast popularity with the general public (e.g. in the UK over 60% of adults are said to read their "stars" first on opening a newspaper, a slightly higher proportion in the USA), there is much argument about the validity of sun sign astrology, particularly amongst astrologers of different persuasions. The more traditional the astrologer, the more likely they are to dislike sun sign astrology.
For example, each person also has a Moon sign, a Mercury sign, a Venus sign etc, and these do NOT conform to the above dates, and change greatly from year to year. Sun-sign astrologers are unable to take these and many other variables into account.
Sun signs are held to represent certain sub-sections of the population. Research by insurance companies within their own massive databases is reported to have revealed differences between the relative safety on the roads of people born at different times of year. The Sunday Times, September 14, 1997 mentions this research and also the remarkable results of a marketing company analysing shopping habits by zodiac sign.
The Swiss mathematician and astrologer Gunter Sachs did some extensive research on massive samples (using nearly a million data sets) and demonstrated striking differences between people of different zodiac signs. These results were reported in newspapers all over the world, for example the Daily Mail, November 6, 1997. Link to that report