Magic is a type of energy and must, therefore, be converted from other energies, not created. Generally, this is from chemical energy, so a magical/caster eats food, stores the energy, converts it to magic to cast/use. 

Using it relies on belief, so for example if a caster believes they must cast spells in Latin, they cannot cast spells in other languages until they realise that they can. This means that, since most m/cs keep to their own social circles, magic use can vary and each user has their own limitations based on their personal beliefs and experiences. 

A magical is something that uses magic e.g shapeshifters. They can convert and use magic energy naturally, but cannot extend or "cast" it beyond themselves.

A caster is someone that casts magic, which could be on themselves or others. They can convert magic energy, but rather than using it in a way inherent to their biology consciously choose how to manifest it.

Both casters and magicals must have the natural ability to convert magic, although this can be artificially created by a caster (with difficulty).