In Defiance there are no levels like in World of Warcraft as such. You will not work your way through levels, level up and then be able to customize the type of character you play as such as Tank, DPS or Support.
Defiance is far more open ended meaning you will be able to adopt pretty much any role you want such as solo, PVP, tanking etc… from your weapon loadout, EGO powers and perks.
So that is the first thing you should know when you are starting out. Of course as you go through the game and complete missions you will get more and more EGO points and then you will be able to select the perks that will compliment your EGO powers and you can make some interesting hybrid builds if you so wish.
So at first glance Defiance is just another shooter but when you start looking into the EGO powers and the grid you use to select them as well as the hundreds of combinations you could have it starts getting a lot more complex quickly!
Then you can add the weapons to the mix and the fact that each one will behave differently, has strengths and weaknesses and you can customize most of them with mods.
However the main thing that will govern your process in ALL of these parts is your EGO rating. If there were a leveling system your EGO rating would be a measure of your progress.
The EGO rating you achieve at max ‘level’ varies depending on the weapons and vehicles you have and what level you have got them too as well as your EGO points. It is around 4000 though.
The main parts of the game that will bump your EGO rating up are:
Weapons & Vehicles – There are 20 levels on each and a fully levelled weapon or vehicle will get you 76 EGO Rating.
EGO Points – You have 80 perks and you will need 3 EGO points to unlock a perk, and 5 points to unlock a main EGO power.
Each EGO power that you max out will give you 50 EGO rating and each maxed out perk will give you 30 EGO rating.
You will also get your EGO rating up by completing missions and Pursuits.
The best thing is to focus on the main missions if you want to make fast progress through the game at the start.
If you REALLY want to make fast progression through the game check out the Defiance Guru guide and start dominating: http://www.defianceguru.com/blog/defiance-beginners-guide/ego-rating-in-defiance-explained-and-how-to-level-up/