What are angels?
Angels are highly spiritual beings. God created angels as our guides, protectors and helpers. He uses them as his messengers. They are not the same as humans who are less highly evolved spirits.
Everyone and everything is made up of vibrations, that is why humans are heavy in vibration and thus can be seen and felt. Angels on the other hand, have a higher, faster vibration and that is why they are usually not visible to humans.
No, humans cannot become angels when they cross over, although you may feel your loved one's presence from time to time. It is possible that they could return as a guide and work with you. There are two recorded instances however, where Archangel Metatron and Archangel Sandalphon lived on earth as Enoch and Elijah respectively and through their ascension became archangels.
There are instances where angels have come to earth to live as humans and to experience life on earth. Their life paths are extremely difficult and more can be read on them in Doreen Virtue's book, Earth Angels.
Angels are in many religions and are spoken about in Judaism, Christianity, Hindu and Islam. They don't belong to any particular sect and are non-denominational. So it's not necessary to change your views or beliefs to work with them.
Angels are also androgynous, meaning they are neither male or female, but have both aspects in perfect balance. However, they accept that we perceive them to be either sex as the human race like to slot things into categories.
The times that angels appear to us as human can also happen. It simply means that they lower their vibration to appear to us as human, however, the appearance normally does not last for an extended period of time.
You needn't have training, be a saint or be perfect, or engage in religious work to communicate and work with angels. They help everyone who calls on them. Please note though that they don't have free will and will not act without your appeal, except if you are facing death and it is not your time to cross over. Then they will step in and take preventative measures.
They help us to understand that we are here for a purpose and that we need to work in co-operation with them and other humans. This opens us to gratitude and that is when the miracles occur.