So, Spider-Man is now in marvel world and will be seen in some upcoming marvel movies. I am not going to write anything on this topic because it is already explained in detail on marvel's blog.
Spider-Man is actually coming back to Marvel because they created this character. Back in 90s marvel sold movie rights for this and some other characters. Some of them were later acquired back and now its turn for the Spider-Man. Spider-Man character is not being aquired by marvel. Sony pictures keep the rights over Spider-Man movies.
Image source: marvel