Although Angels require the permission and consent of the human host for possession to be possible, whereas Demons can involuntary claim a human's body without the consent of the human host.
The most unique element about angels (in Supernatural) that isn't in typical lore is the notion that they have to possess. They control a human host in much the same way demons do. The only difference is that angels have to ask permission and demons don't. That provided us with all sorts of morality plays to work through because angels are nominally good, but they are putting human victims through this horrific experience just because the humans were devout enough to give their will away.
– Eric Kripke, Official Website
Key terms pertaining to Manifestation
Possession - The term "Possession" describes the process in which a supernatural entity takes over a human's body voluntarily or involuntarily depending on the entity, effectively controlling the thoughts, speech and actions of the host.
Vessel - The term "Vessel" describes a human's body that an Angel occupies or inhabits after bearing the consent or permission given by the host, as it is required for all angels.
Enochian Chant - The term "Enochian" refers to many things concerning angels in general. There is a specific Enochian chant that can forcibly remove Angels from their Vessel, sending them back to Heaven.
Binding Link - The term "Binding Link" refers to a specific bond or connection that Demons can use to bind or lock themselves inside a human's body, in which case exorcism proves ineffective against the demon within the host.
All of the information documented below only describes the first time a particular important Angel or Demon manifests on Earth using a human host.