Perpetual vs Annual?

I searched all over the website and even through the forums, but for the life of me, I can not find a description of the differences between and Annual license and a Perpetual License. I can only assume that Perpetual, as the name suggets, means I pay once and never have to buy upgrades. I assume I will always have access to the latest version for my Application? And I assume that would apply to nodes as well? Can someone quickly confirm or explain the differences? Or point me to where the two purchase models are explained.

Thank you!
Dan