The Viral is a beginning state that's the infected turning into. The Viral is someone who just got bitten and just got infected, so they still show traces of humanity, and at this point they are very quick, very dangerous—they avoid the sun at all costs.
But after some time in that state, they turn into Biters, which are basically regular zombies; slowly moving infected that, if able, will try and grab you and infect you or eat you, but the sunlight is dangerous to them, and they are not as quick and as agile as the Virals.
The Degenerate is a final stage of 'evolution' for the Virals; decaying zombies that are very weak with pieces of flesh falling off of them. The more sunlight Biters get, the weaker they will become. In other words, the longer they live with the infection, the weaker they will get unless they can avoid the sun's UV rays.
The Demolisher is the largest infected type encountered by the player. Its strengths lies in its ability to break down walls and lift and toss vehicles across far distances.