Lam Rim (Tib: lam "path", rim "stages") is the presentation of Buddhas doctrine of Tibetan Buddhism based on extensions of Atisha's root text A Lamp for the Path.
The Kadampa monk and student of Milarepa, Gampopa, introduced the Lam Rim to his disciples as a way of developing the mind gradually. Gampopa's work on Lam Rim is known as The Jewel Ornament of Liberation: The Wish-Fulfilling Gem of the Noble Teachings and is studied to this day in the various Kagyu schools of Tibetan Buddhism founded by his disciples.
As Tsongkhapa's Gelug school is derived from and mainly based upon the Kadampa school, it comes to no surprise that Tsongkhapa wrote one of his masterpieces on Lam Rim: The Great Treatise on the Stages of the Path of Enlightenment (Tib. Lam Rim Chen Mo) which has about 1000 pages, and is primarily based on literary sources. There is also a medium Lam Rim text by Tsongkhapa (200 pages) and a short one, called Lam Rim Dudon (Tib.), which is often recited daily by Gelugpas and is about 10 pages long.
Excluded from this division were individuals whose motives revolve around material benefits within this life.
Persons of low motive are searching with all means for happiness within Samsara; their motive is to achieve high rebirth, typically in a heaven. Buddhists traditionally considered this domain included followers of most non-Buddhist religions.
Persons of medium motive are searching for their own peace and abandon worldly pleasure. This includes the paths of the PratyekaBuddhas and SravakaBuddhas.
Persons of high motive seek (based on their insight of their own suffering) by all means to stop the suffering of all beings. This is the Mahayana paths of the SamyakSamBuddhas.
The Lam Rim of Atisha starts with Bodhicitta, the altruistic mind, followed by taking the Bodhisattva vows, whereas Gampopa's lam Lam Rim starts with the Buddha nature, followed by the Precious Human Life and Tsongkhapas texts start with reliance to a master, followed by the Precious Human Life, and continues with the paths of the low, medium and high scopes.
Gampopa and Tsongkhapa expounded the short root-text of Atisha into an extensive system to understand the entire Buddhist philosophy. In this way, subjects like karma, rebirth, Buddhist cosmology, the practice of meditation, up to and including tantra are gradually explained in logical order.
The Lam Rim teachings are in essence very similar to the Lamdre teachings of the Sakya school of Tibetan Buddhism.