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Angels appear in the Bible to Hagar in the desert (Genesis 16:9) to Abraham telling him not to follow through with sacrificing his son Isaac (Genesis 22:11). They feature in Jacob's dream of the ladder stretching from earth to heaven with angels ascending and descending on it and appear to Moses, Balaam and Joshua.
The Midrash on Parsahat Vaera stipulates that each angel can only carry out one task, therefore three must be sent; one to heal Abraham, one to announce that Sodom will be overthrown and one to bring the good news that Sarah will become pregnant.
Talmudic and Kabbalistic writings embellish hierarchies and systems of angels that we can't begin to go into.
I admit that I don't really understand angels. They make most sense to me when they are compared to people in ways that are educational. For example, in the Talmud Shabbat 88a, the angels ask why humans and not them are chosen to receive the Torah; God points out that the angels, not having a body, don't need Torah. They don't have parents to honour, they don't do physical work that they need to rest from, they are not subject to jealousy and desire etc. Torah comes to sanctify the physical body and world which is our domain, not the angels'.