Adventurers seek gold, glory, justice, fame, power, knowledge, or maybe some other goal--perhaps noble or perhaps base. Each chooses a different way to attain those goals, from brutal combat power, to mighty magic, to subtle skills. Some adventurers prevail and grow in experience, wealth, and power. Others die.
Your character's class is his or her profession or vocation. It determines what he or she is able to do: combat prowess, magical ability, skills, and more. Class is probably the first choice you make about your character--just ahead of race or perhaps in conjunction with that decision. The class you choose determins where you should best place your character's ability scores and suggests which races are best to support that class choice.