Creo is the new name given to Pro/E with additional functionalities and few new modules.
Creo 1.0 is the latest version of Pro/E.Before Creo 1.0, it was Pro/E Wildfire 5.0.
You can open lower version files of Pro/E say WF 5.0,4.0,3.0,2.0,1.0......on Creo 1.0.
But latest version files can't be opened on Lower Version of Pro/E.
I hope this gives the answer to your query.
That answers the second half of my question. But when you say "latest version files can't be opened on Lower Version of Pro/E", just to clarify, does that also mean I can't use the "save as" function in Creo 1.0 and save them (or export them, whatever you want to call it) as Pro/E WF2/3/4/5 files?
Mr. Mahanta has it correct. Don't bother with the "Granite Based Cross Release Interoperability". It's more of a hassle than it's worth.
Newer releases of Pro/E (and Creo) can always open the older ones. With limited exception, you can't go the other way around. Wildfire 2 isn't going to open Wildfire 4 parts. As the software matures and gets better, you can't go back. New features are always being added that didn't exist in earlier versions.
The best you can do is export a STEP or other solid model file and re-import it back into an earlier version of Pro/E. This will preserve the size/shape of the model but all intelligence will be lost.
If you're looking to jump into Pro/E, evaluate any customers you have,. If they need Wildfire 3 models, jump in there. If your customers aren't using Pro/E... or if they just want a "dumb" STEP model or other solid model format, go with Wildfire 5.0 (Creo Elements Pro/5.0). This is the latest, most stable platform to work from.
Personal advice... don't go with Creo 1.0 yet. It's too new and there are still some kinks being worked out. Starting with Creo Elements Pro/5.0 (formerly called Pro/ENGINEER Wildfire 5.0) is the best bet in my opinion.