Kaal sarp yog is formed when in an horoscope, all the seven planets comes between or hemmed between rahu and ketu. Rahu is considered serpent's head and ketu is dragon's tail. So the presence of kaal sarp yoga in an horoscope is considered particularly harmful as all the planets are surrounded by the serpent and thus loose their good impacts to a great extent which will make person less lucky, creates problems and delays in many important aspects of life.
When all the seven planets are on the same side of rahu and ketu axis and not a single planet is on the other side, the full kaal-sarp dosha is formed. If even a single plane is on the other side of this axis, then it is not a kaal sarp yog. In this case, partial kaal sarp yoga is formed which is a kaal sarpa yog of less intensity.
Having kal sarp yoga in horoscope doesn't mean that person will be totally unlucky and devoid of all the good things of life. The overall results differ from person to person and it depends on other yogas in the particular horoscope. For example, if a person is having strong Raj yogas present or having four planets dig-bali or having two to three escalated or swagrehi planets in horoscope alongwith kaal sarp dosha, then the results of kaal sarp yoga will be minimized in this case by the strong good planets or yoga and major ill impacts will only be applicable in the major/minor period of rahu/ketu.
The general impacts of kaal sarp dosha are following: