The Overworld is the main, Earth-like, dimension. Most of Minecraft Diaries, and other of Aphmau's series takes place here.
The Wyvern Dimension is where mainly all the Wyverns and the Elder Wyvern come from. Wyverns from this dimension currently Exist on an uncharted island but sadly one has passed(R.I.P Ungrith)
Irene's Dimension is a very rare thing to come about. Not everyone know's about the dimensions existence. It can be accessed through the Amulet and a portal, but the portal costs something important to a person to be sacrificed to open it (ex. Immortality:Zoey)
The Nether Portal Leads to The Nether. It is known that you should never tamper with the outside of the portal if you want to break it. The only way to break is by going inside, then breaking it. An active Nether Portal exists in the ruins of The Southern Wolf Tribe.
The Wyvern staff is an item that teleports you to the Wyvern Dimension. It's a very hard item to get this item because sacrifices need to be made.
The Amulet lets the person travel to any Dimension/Realm of their choice. They are very hard to find/create, etc. The one that created the 15 year time-skip to the Irene Dimension was created by a cursed necklace and a Zonorelian Pendant. The two items in contact created the amulet.
The Irene Portal allows you to acces Irene's Dimension. It can only be opened when you sacrifice something important to a person.