Sure!
So you have Elements, the regular game, what you know. When development went into a lull, a few people made a spinoff game,
OpenEtG, with an open-sourced code. This version has many similar things to the regular game, but has more cards, some balance changes on existing cards, and a few other modes. As well as a new False God roster (named Demigods) to accommodate the new cards.
OEtG was not made in Flash, while regular Elements is. This means, when Flash gets cancelled, most people will be looking towards OEtG as the major game of choice for the community. It does lack in some areas, but those would hopefully be ironed out before the "Flashpocalypse".
Vanilla OEtG is slightly different. It is basically original Elements game, but with the "skin" of OpenEtG. It is a dedicated PvP server, so when the PvP servers for Elements go down, these can be used as a backup. PvP is also faster, which makes it nice for testing decks in a PvP environment.
If you have any more questions, feel free to ask