Complex sentence: Characters start classless, and gain the ability to acquire a class (as well as switch classes and revert to classlessness) at a certain point in the storyline, each class provides bonuses and, in many cases, penalties to a character's base stats.
Split into simpler sentences: Characters start classless, and gain the ability to acquire and switch classes during the storyline. Classes provide bonuses and, in many cases, penalties to a character's base stats.