I actually had to make it up. The game just gives them all spell casting. It might as well be “mysterious forces”.
Since you don't apparently own a PHB, let me help
PHB:
In the worlds of D&D, words and music are not just vibrations of air, but vocalizations with power all their own. The bard is a master of song, speech, and the magic they contain. Bards say that the multiverse was spoken into existence, that the words of the gods gave it shape, and that echoes of these primordial Words of Creation still resound throughout the cosmos. The music of bards is an attempt to snatch and harness those echoes, subtly woven into their spells and powers
...
Magic is a part of every sorcerer, suffusing body, mind, and spirit with a latent power that waits to be tapped. Some sorcerers wield magic that springs from an ancient bloodline infused with the magic of dragons. Others carry a raw, uncontrolled magic within them, a chaotic storm that manifests in unexpected ways.
...
A warlock is defined by a pact with an otherworldly being. Sometimes the relationship between warlock and patron is like that of a cleric and a deity, though the beings that serve as patrons for warlocks are not gods. A warlock might lead a cult dedicated to a demon prince, an archdevil, or an utterly alien entity—beings not typically served by clerics. More often, though, the arrangement is similar to that between a master and an apprentice. The warlock learns and grows in power, at the cost of occasional services performed on the patron's behalf.
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Wizards live and die by their spells. Everything else is secondary. They learn new spells as they experiment and grow in experience. They can also learn them from other wizards, from ancient tomes or inscriptions, and from ancient creatures (such as the fey) that are steeped in magic.
HammerMan: