The four stages of enlightenment in Buddhism are the four degrees of approach to full enlightenment as an Arahant which a person can attain in this life. The four stages are Sotapanna, Sakadagami, Anagami and Arahant.
The Ordinary person
An ordinary person, or puthujjana (Pali; Sanskrit: pṛthagjana) is trapped in the endless changes of saṃsara. Doing good or evil as influenced by his desires and aversions, an ordinary person is born in higher or lower states of being (heavens or hells) according to their actions. As these persons have little control over their minds and behaviors, their destinies are haphazard and subject to great suffering. The ordinary person has never seen and experienced the ultimate truth of Dhamma, and therefore has no way of finding an end to his predicament.
The Noble persons
One who begins sincere training on the Buddhist path (known as Sekhas in Pali or those in training) and experiences the truth to the extent of cutting of a number of the ten mental fetters (Pali: saṃyojana), becomes an ariya puggala (Pali; Sanskrit: āryapudgala): a "noble person" who will surely become an Arahant in the near future (within seven lives). Their specific path is governed by the degree of attainment reached.
"Among whatever communities or groups there may be, the Sangha of the Tathagata's disciples is considered supreme... Those who have confidence in the Sangha have confidence in what is supreme; and for those with confidence in the supreme, supreme will be the result." 
The Sangha of the Tathagata's disciples (Ariya Sangha), i.e. the four [groups of noble disciples] when taken as pairs, the eight when taken as persons. The four groups of noble disciples (Buddhist Sekhas) when taken as pairs are those who have attained:
(1) the path to stream-entry; (2) the fruition of stream-entry;
(3) the path to once-returning ; (4) the fruition of once-returning;
(5) the path to non-returning ; (6) the fruition of non-returning;
(7) the path to arahantship ; (8) the fruition of arahantship.
Taking each attainment singly gives eight "individuals."
The first stage is that of Sotāpanna (Pali; Sanskrit: Srotāpanna), literally meaning "one who enters (āpadyate) the stream (sotas)," with the stream being the Noble Eightfold Path regarded as the highest Dharma. The stream-enterer is also said to have "opened the eye of the Dharma" (dhammacakkhu, Sanskrit: dharmacakṣus).
A stream-enterer is guaranteed enlightenment after no more than seven successive rebirths, and possibly in fewer. The stream-enterer can also be sure that he will not be reborn in any of the unhappy states or rebirths (an animal, a preta, or in hell). He can only be reborn as a human being, or in a heaven.
The stream-enterer has attained an intuitive grasp of Buddhist doctrine (samyagdṛṣṭi or sammādiṭṭhi, "right view"), has complete confidence or Saddha in the Three Jewels of Buddha, Dhamma, and Sangha, and has good moral behaviour (Sila).
The second stage is that of the Sakadāgāmī (Sanskrit: Sakṛdāgāmin), literally meaning "one who once (sakṛt) comes (āgacchati)". The once-returner will return to the human world only one more time, and will attain Nirvana in that life.
The third stage is that of the Anāgāmī (Sanskrit: Anāgāmin), literally meaning "one who does not (an-) come (āgacchati)". The non-returner does not come back into human existence, or any lower world, after death. Instead, he is reborn in one of the worlds of the Rūpadhātu called the Śuddhāvāsa worlds, or "Pure Abodes", where he will attain Nirvāṇa; Pāli: Nibbana; some of them are reborn a second time in a higher world of the Pure Abodes, but in no case are born into a lower state.
The fourth stage is that of Arahant, a fully enlightened human being who has abandoned all fetters, and who upon decease (Sanskrit: Parinirvāṇa, Pāli: Parinibbāna) will not be reborn in any world, having wholly abandoned saṃsāra.