In Theosophy and modern New Age beliefs, Lord Maitreya is one of the many names given to the Lord of the Ascended Masters. Often the name Sananda is used as well. As Lord Sananda, he is said to be in charge of the Spiritual Guidance of the Ashtar Command, also known as The Great White Brotherhood.
Jesus Christ would be one of his 3rd dimensional incarnations. Or rather, he is believed to have been a walk-in, where he and the physical Jesus (Yeshua Ben Youssef) shared the same body. The same is said of Krishna, and of the Buddha.
In certain branches of Buddhism, the name Maitreya is (a transcription of) the name, given to the Buddha in the event of his Second Coming.