There are four seasons available to choose from. Every class/season combination has it's own skill set as all four seasons have unique characteristics. Players can switch seasons by using portal to the right of the Seasonal Shrine in Plimouth for 1000. Every time a class level is gained a level of the currently used season goes up as well. Used season might affect your damage - for details see the Combat Tips & Tricks article
Spring classes are based around speed, change and gut reactions; as such, they tend to focus on self enhancement and attacks that do damage over time.
Summer is the most grounded of the four seasons. Their classes reflect protection and healing. They tend to focus on outlasting their opponents rather than killing them quickly.
The essence of winter is control and area of effect attacks.