Charvaka

Charvaka (Sanskrit: चार्वाक; IAST: Cārvāka), also known as Lokāyata, was an ancient school of Indian materialism. It rejects religious authorities like the Vedas and opposes Hinduism. It was an materialistic and atheistic school of philosophy. Along with Buddhism and Jainism, it was one of the three major nastika schools of Indian philosophy.
Teachings.
The school holds direct perception, empiricism, and conditional inference as proper sources of knowledge, embraces philosophical skepticism, and rejects ritualism and supernaturalism. It also rejects supernatural concepts like God, the soul, and metaphysical concepts like the Afterlife (reincarnation) and samsara. It used to be a popular belief system in ancient India, although Charvaka doctrine disappeared by the end of the medieval period. Its onetime importance is confirmed by the lengthy attempts to disprove it found in both Buddhist and Hindu philosophical texts, which also constitute the main sources for knowledge of the doctrine. Charvaka also rejects the caste system.
Lost teachings and texts.
Much of the primary literature of Charvaka, and the Barhaspatya sutras, were lost either due to waning popularity or other unknown reasons. However, there is text that may belong to the Charvaka tradition, written by the skeptic philosopher Jayarāśi Bhaṭṭa, known as the Tattvôpaplava-siṁha, that provides information about this school, albeit unorthodox. Charvaka rejects caste system.
During the Sramana movement in the first millennium BCE, when Buddhism was established by Gautama Buddha and Jainism was re-organized by Parshvanatha, the Charvaka philosophy was well documented and opposed by Hinduism and the other two "nastika" schools.
Differences between other non-Vedic schools.
Unlike other major Nastika religions or philosophies like Jainism and Buddhism, which are dharmic philosophies, Charvaka is not a dharmic philosophy because it rejects basic concepts of dharma like karma and moksha. Charvaka is an Adharmic philosophy, so other religions viewed the Charvachic teachings negatively. Hinduism opposed the teachings of Charvaka because Charvaka rejects the authority of the Vedic texts or any sacred scriptures and opposed brahmanism.
Sometimes it is considered as a part of Hinduism (Hindu philosophy) as the word Hindu is an exonym and was a geographical identity in ancient times but most scholars reject it as a part of Hinduism since it's also called "sanatana dharma" and Charvaka rejects basic concepts of "sanatana dharma" like rta, the authority of sacred text like the Vedas (which is considered as an authority in Hindu philosophies). It also rejects the existence of an Atman or a Brahman (om).