When you think of angels in Christianity and in Judaism, Devas are a little similar to angels. According to the scriptures, Devas are basically beings who live in a certain heaven realm. We can't see them, but they are actually living in the same plane of existence.
The Thai people often have a tree in their yard which has fancy ribbons around it. Traditional Thai doorways have a bar across the bottom so that you have to step over it in the doorway. The reason for the fancy ribbons and things around trees is because some Thais believe that Devas live around trees. The reason for the bar in the doorway is because some Thais believe Devas live in a doorway area. By having to step over the doorway right at that point, you don't step in the Deva's home. These are the belief systems.
Now, the Buddha said that Devas can sometimes affect your life and there are some forms of Devas that aren't always very friendly to humans. But in general Devas are friendly to humans.
It's a belief theory in Buddhism, which may be true.