When you start, it looks like Dungeon Runners has classes. You start as a mage, ranger, or warrior. Townston has three trainers, one for each class. You quickly learn that anyone can learn any skill. Your starting class only determines which four skills you start with. The elemental resistance buffs are spread across the three classes, and one of the best mage AOE damage spells is a ranger skill.
Your limit is that you can have ten skills active at a time. Think Guild Wars, only you can change them mid-dungeon. You have one hotbar, with 1-8 plus two mouse keys. This includes your passive skills, which only work if you have them on the hotbar. Changing skills activates their cooldown timer, which is irrelevant for attacks but keeps you from having all the buffs active at once.
A respec is just a button that resets your stats (and costs you gold). Re-allocate your stats, change your skill bar, and put on some new equipment. Congratulations, you are a ranger today.