I've heard that they are something like sentient trees but I have also heard somewhere that thy are an older form of argonians. So....what are they?!

They're an intelligent strand of trees that seemed to have been around since the beginning of Nirn. They're old enough to have been around the same time or older than the Ehlnofey (proto-mer and man), as http://www.imperial-library.info/content/annotated-anuad.

http://www.imperial-library.info/content/forum-archive-mark-nelson

And you should get a hold of two TES books, as there is some interesting info about the Hist there. They also recognize that Sithis is the push that caused the start of all.