The term angel is used in a wide variety of meanings in different contexts. Generally the term means a spiritual, non-physical being that belongs to some spiritual hierarchy or performs some specific function for a higher spiritual authority such as God. An angel is generally understood as being benevolent.
George Gurdjieff makes some mention of angels and archangels in the "diagram of everything living". These seem to correspond to 'planetary' and 'solar' beings, or fourth and sixth density in the system presented by the Cassiopaeans and Ra.
The New Age movement has made angels a sort of fashion. There are books and guided meditation tapes and such for contacting one's guardian angels, communicating with angels and so forth. Much of this seems to simply stimulate people's imagination towards further wishfulness. The problem is not that genuine communication with higher beings cannot take place, but rather the indiscriminate approach and the assumption that one would be prepared for such by attending a week-end seminar.
The Cassiopaean material defines an angel as being a sixth density service to others being. An actual direct encounter with one is, according to the source, of such a nature as to leave no doubt to its authenticity. However, fourth density service to self beings may also present themselves as angels, among other deceptions. However, fourth density STS beings tend to give themselves away, as there seems to be a sort of spiritual law that such contacts cannot entirely disguise their nature. Violations of free will, demands of worship, flattery, or cultivating a sense of chosenness are examples of signs of a negative influence.
The Cassiopaean material states that humans do not have any dedicated "guardian angels" or similar; rather, all are responsible for themselves, although they may receive help if they ask in the appropriate manner.