Can someone please explain the differences between using fpGUI and MSEide/MSEgui with Lazarus?
Both are excellent gui frameworks. A lot of effort and love went into creating them, and to fulfil the needs of the origin creators. And yes they are suitable for beginners too. fpGUI has a more similar (not identical) API to Delphi's VCL or Lazarus's LCL, so might be easier to pick up, if you are used to the former two. MSEgui's API is somewhat different. But make no mistake, different doesn't mean bad. They are equally capable frameworks and both have a small learning curve (just like any other gui toolkit).
> Which is better maintained?
Granted, I have taken a few years off, to focus on my new career and getting to grips with Java. But I'm back to maintaining and improving fpGUI now. So yes, both are well maintained.
> Do they both work with fpcupdeluxe?
It's just FPC, so yes.
> Do they both support the same platforms?
yes
Hope that helps. :-)