"ray flower" is the petal-like flowers at the edge of the flower head, right?

Okay, then I have a question, is there a difference between "ray flower" and "petal" ? Does "ray flower" actually refer to "petal" ? ( I mean, they are the same parts) Or, in fact, they are complete different parts of the flowers?

Botanically, yes. The term "ray flower" is usually associated only with members of the Asteraceae (or Composites). Members of this family have flowers composed often of both ray flowers and disc flowers, sometimes only ray flowers and less frequently, only disc flowers. A sunflower would be an example of the first (the disc flowers being the inner eye and the rays the outer petals), a dandelion or marigold would be an example of the second and a thistle would be an example of the last. While it may look very much like one, a ray flower is not technically a petal as it is a complete flower bearing necessary sexual parts.

What we most commonly refer to as a petal (or sometimes tepal) is the usually brightly colored part of the corolla immediately adjacent to but not part of the reproductive organs. Or in other words, only a part of a flower.

Thanks a lot !