As I said in the post where I talked about all the classes the Siren class is the skillful class of Borderlands 2. It has different abilities and the upgrades let you buff those abilities to make them even more effective. The Siren class is the support class of Borderlands 2 allowing you, for instance, to heal other players by shooting them.
Maya is the generic Siren Class name in Borderlands 2, but you can customize the name and some aspects of the appearance of your character. You have to do that before you start the game and I think your choices are permanent.
If you prefer to listen to this post check the video at the end of this post.
The motion skill tree lets you buffs your abilities (mostly Phaselock). It also gives buffs to other skills whilst you use Phaselock which help with dealing out damage.
Harmony (Healing and Support)
Harmony is the support skill tree of the Siren class. If you ever find yourself playing with other people these perks are a must. They offer healing on shooting friendlies and easier revival out of second chance. But you can also buff yourself using motion which can heal yourself and buff your gun damage and fire rate.
Cataclysm (Dealing Damage)
Cataclysm is designed to maximize the amount of damage you can do to enemies, mostly through the different weapon types like corrosive or fire damage. Most of the ways you can deal damage, be it guns or melee, are buffed with various upgrades in cataclysm.