What are the pros and cons for each of the classes?

Raider:

Strongest attacker and can raid gold (using class legion) from other players.

Lower production rate for rss.

Good for regular players who like to fight.

Farmer:

Highest production and gathering rates.

Slow marching and weaker at attacking enemies.

Good for more idle / peaceful players

Trader:

Higher gold production, can send rss to alliance members.

High losses if attacked.

This is a Support class option. For players who want to help others.

Builder:

Buffs to build speeds and requirements.

Useful when you start to help you get leveled up more quickly but once you reach level 13 or so you should pick one of the other classes depending on your play style.

You can see more details at Classes and How to change class.