Are you kidding me? You don’t even know* that* much about it?
Sun sign astrology is not a different form of astrology, its the term used for the sort of drivel you see in the daily paper, and in the godforsaken paperback crap you buy in an airport gift shop. It is the lowest of low. It is frequently titled as a form of “horoscope”, it is nothing of the sort. It is the sign that the sun occupies, hence, one is a Taurus, a Gemini, a Cancer, and so forth. (Other astrologers start with Aries, but they are too belligerant, so I’ve decided to ignore them.)
An actual “horoscope” is a “snapshot” of the configuration of the major planets (with Sun and moon defined as “planets”). It considers their placement relative to a given sign (Mercury in Taurus, the mind is solid and practical, in Gemini, quick and verbal…etc.) It also compares angular relationships, a square (90 degrees) relationship is vexing and unstable, a trine (120 degrees) is easy and stable. So, in a previous example: a Mercury in Gemini square to Mars would be quck, verbal, but prone to ill-temper, sharp-tongued, acerbic. If this Mercury placement is in the 6th house, the house of employment, you’ve got someone who makes their living in a mental occupation, involving a lot of talking, who is likely to have difficulty on the job due to bad temper. Prone to accidents due to haste, spends more time in a hospital for injury than illness. Needs to chill, seriously, dude.
And this is just for starters. It gets pretty complicated.
Quick and dirty, that’s what a horoscope is. A “horoscope” that only considers the sun sign is too stupid to be mentioned in polite company. Of course, what Dr Forer thought a horoscope was, we know not, but await with bated breath.