Astrology divided into two parts - Calculative and Predictive
A) Calculative Part: In this part birth details of a person- date of birth, time and place are gathered and then position of planets at the time of birth of a person is calculated using some astronomical calculation.
A Panchang or ephemeries was used for this purpose earlier but nowadays many computer softwares available in market or on internet to do this job. A horoscope is generated after filling some details which consist so many divisional charts and dashas, etc.
B) Predictive Part: In predictive astrology, a person consult with an Astrologer. Astrologer analyse the person's horoscope and tries to predict about the person's life from various point of views like education, job, career, marriage, health, child birth, financial status etc.