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.
Theosophy, anthroposophy and various Gnostic teachings have detailed descriptions of various categories of angels, entire 'heavenly organization charts.'
Gurdjieff makes some mention of angels and archangels in the diagram of everything living. These seem to correspond to 'planetary' and 'solar' beings, or 4th and 6th density in the Cassiopaean/Ra system.
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 would never take place, rather the problem is the indiscriminating approach and the assumption that one would be prepared for such by attending a week-end seminar.
The Cassiopaea material defines an angel as being a 6th density STO 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, 4D STS may present themselves as angels also, among other deceptions. However, 4D STS tend to give themselves away, as there seems to be a sort of spiritual law that such contacts cannot entirely disguise their nature. Violation of free will, demands of worship, flattery or cultivating a sense of chosenness for example can be signs of a negative influence.
The Cassiopaea material states that humans do not have any dedicated guardian angels or such, rather all are responsible for themselves, although may receive help if they ask in the appropriate manner.