Hi adventuregamers!

I was just wondering, what is the difference between these two... let's take 2 games, as an expample:
1. Undercover: Operation Wintersun (which is mentioned 2.5D)
2. Secret Files: Tunguska (which is mentioned Slideshow)
I have played both two demos and my opinion about their graphics was that they could be from the very same game. I really didn't notice anything that would change the presentation... So you wiser

Tell me what is 2.5D exactly?