Avalokiteshvara means "the lord who sees the sufferings of sentient beings". "Avalokita" means "one who looks down", "ishvara", which is changed to "eshvara" due to sound combination, means "lord or master".
According to Ven. Samdhong Rinpoche, (Sarnath, August 1990), Avalokiteshvara is a Bodhisattva who adopted the Bodhicitta of the third kind. The third kind of Bodhicitta is called "shepherd-like Bodhicitta". "The sheperd will always take care that no sheep remains behind and each reaches safe shelter". This means that the Bodhisattva promises to guide all sentient beings to Buddhahood and only thereafter will he achieve his own Buddhahood. Up to this time he will practice the bodhisattvacarya (the activities of a Bodhisattva).
Samdhong Rinpoche says that this is actually symbolic. Avalokiteshvara has already achieved Buddhahood countless aeons ago, but pretends to be still a Bodhisattva so that he can come closer to sentient beings. There are many sentient beings who do not have enough merits to see a Buddha or the manifestation of a Buddha but they have the merits to see a Bodhisattva or the manifestation of a Bodhisattva. For that reason Avalokiteshvara always remains a Bodhisattva.
Being a Bodhisattva he is receiving teachings from various Buddhas such as Gautama Buddha who is the 4th Buddha in this aeon. Avalokiteshvara became one of the eight chief Bodhisattva disciples of the Buddha(the others are Manjushri, Vajrapani, Maitreya, Samanthabadra, Kshitigarbha, Sarvanikaranaviskambini and Akashagarbha). In fact he is the most important of the eight chief Bodhisattvas. He received complete Mahayana teachings, especially the prajnaparamita or higher wisdom teachings. Therefore the tradition of the paramitayanasadhana (training and meditation in the 6 perfections) has been started by Bodhisattva Avalokiteshvara.
There are many sutras in which Avalokiteshvara participate as a main disciple or questioner starting an argument or discussion. As a Bodhisattva he participated a great deal to receive, promote and to spread the teachings of Buddha Shakyamuni.
Avalokiteshvara embodies compassion, which along with wisdom, is one of the two main characteristics of the awakened mind of a Buddha.
Until the 10th century, Avalokiteshvara has been portrayed as male but in East Asia, during the Sung Dynasty, the image of a female in white robe (Pai-i Kuan Yin) became popular. To help beings, the Buddha manifests in any suitable form. In old China, plaqued by too many princely warriors, the image of a young prince would not have been as effective as the motherly caring pure being of Kuan Yin.
Avalokiteshvara is regarded as an emanation of Amitabha Buddha and helps his teacher to propagate the Dharma in the Pure Land. One of his aspirational prayers is that "whoever hears or recites his name and whosoever recites his mantra must attain the awakening of Bodhicitta and must attain great benefit from it."
Tibetans regard Avalokiteshvara as their patron deity because he took that region as his own personal field for his bodhisattvacarya, his activities. And one of his physical emanations is the line of Dalai Lamas.
Who are the other emanations of Avalokiteshvara? It is said that just as the Great Compassion of Buddha emanates as Avalokiteshvara, anyone who develops that Great Compassion becomes automatically an emanation of Avalokiteshvara. Therefore there are many emanations of Avalokiteshvara in the history of Buddhism.