девочка - girl (a child)
девушка - a young women.
There is not an exact border between two. I think after puberty one can be called a "девушка" but she is still a schoolgirl at that time, so a child, "девочка" as well.
Similarly, there is not a border between девушка and женщина, it much depends on how woman feels about this herself.
Девочка can also be used affectionately. Also it happens that a lady in her 60s addresses her friends "девочки" (if their realtions are informal)