There are three important symbols to remember in Dawn of Titans.

Mobile, Ranged, and Heavy.

As an example, Thor is a Heavy Titan. He is a slow Champion Class and has Lightning Elemental abilities. 

And Nergal is a Mobile Titan. He is a fast Infiltrator Class and has a Physical Element ability. 

Lastly, Nephilinis is a Ranged Titan. She is a Ranger Class and has Fire Element ability.