Did Lord Buddha teach, or do Buddhist sects teach, about Ghosts? The existence of Ghosts is taught in many religions. And Buddhist sites (mainly Tibetan Buddhists) claim the existence of "Hungry Ghosts".

However I didn't see sites saying that the Buddha claimed that ghosts exist.

Did Lord Buddha teach about Ghosts, or is it only Buddhist sects which teach that?

Also do ghosts exist here on this earth, do they affect or possess people, are they visible? Or are they only in some other real of existence?
There are many such stories in Petavatthu, which appear in canonical texts. Buddhist Cosmology mentions 31 Planes of Existence. The realm of Hungry Ghosts is one of these realms. Average people cannot see any of these realms other than the parts of the human realm and animal kingdom. This does not mean, what we see do not exit.

Also see:


From preta to hungry ghost by Piya Tan
Who are the hungry ghosts? by Piya Tan