Kaal Sarp forms when all seven classical planets — Sun through Saturn — fall on one side of the axis running between Rahu and Ketu. The image is of the planets hemmed between the head and the tail of a serpent, and the reading is that life runs along a track laid down before it began.
Full, partial, and neither
Seven planets on one side is a full formation. Six on one side with one outside is counted as partial, and is much milder — most astrologers treat a partial as a note rather than a finding. If planets fall on both sides with more than one outside the axis, there is no Kaal Sarp at all.
The twelve types
Which formation applies depends on the house Rahu occupies, and that decides the area of life where the pattern shows: Anant in the 1st, Kulik in the 2nd, Vasuki in the 3rd, Shankhpal in the 4th, Padma in the 5th, Mahapadma in the 6th, Takshak in the 7th, Karkotak in the 8th, Shankhachud in the 9th, Ghatak in the 10th, Vishdhar in the 11th and Sheshnag in the 12th.
Not all twelve read the same. Mahapadma, with Rahu in the 6th, is the mildest — the 6th is a house where Rahu performs well, and the formation gives unusual power to defeat competition. Takshak, with Rahu in the 7th, is the one that most affects marriage, and the texts specifically advise against marrying early with it.
What it actually indicates
The consistent reading across the twelve is that results arrive in bursts rather than steadily. Effort accumulates without visible return for long stretches, then delivers a large result at once. Progress is late but often substantial. It is a pattern of timing more than a pattern of misfortune.
A note on proportion
Kaal Sarp is a late addition to the classical literature. It does not appear in the foundational texts, and the alarm attached to it in popular astrology is not supported by the older sources. It is worth knowing about and worth putting in proportion — a great many successful charts carry it.
Six planets on one side is a partial formation and is read far more lightly than a full one.