The new Spellcaster system allows players to make their Sim into a new life state that can use magic. There are a few differences between Spellcasters and normal Sims, so let's outline them here and give you some information to get started playing them.
Becoming a Spellcaster
There are a total of three ways to become a spellcaster in The Sims 4 Realm of Magic. Let's start with the way that will probably be used by most people new to the pack, which is accessible to any existing Sim Teen or up: Ask one of the three Sages in the Magic Realm.
You can also make a Spellcaster via Create-a-Sim just like any other Occult. You'll start out at Apprentice rank just like a Sim who did the quest.
The finally way to become a caster is to be born one. It may be that you still need to do the quest via one of the Sages or ask your super powerful parents to endow you with magical ability. If you have a Sim born to spellcaster parents, you'll get the Magical Birthright trait. This ensures that you get more Spellcaster Perk Points on rank up and may have some other effects. Maxis have said that Magical Birthright actually grows in power the more generations of Spellcaster a Sim has in their ancestry.
Spellcaster Charge - Not a Typical Mana System
The main difference between Spellcasters and regular Sims is found on the needs panel. There's a new Spellcaster Charge meter up top. Using magic increases charge, causing your Sim to be utilizing more energy when they cast. This growing energy devotes more power to spells they cast but comes with a price - increased failure rates. Each spell you cast adds to the charge, and the amounts may vary depending on the difficulty of the spell (speculation).
Aside from failure rates and increased spell potency, there's a second thing about Spellcasters and the charge meter - when it's full, your Sim is in danger of a new Death by Overcharge. This new death type results in a Ghost with a Trait that makes them immune to another death by overcharge. This may be useful in some cases, meaning the leash is off when a Spellcaster has left the realm of the living.
Spellcaster Charge depletes naturally over time, but you can take perks that help you to manage it. Additionally we know there's a Discharge spell that causes a Sim to release this built up energy. This may be useful if you're near full on charge and want to continue casting. However we don't know if there are any drawbacks to this yet.
It looks like there are at least 6 Ranks to the Spellcaster system in Realm of Magic. The new system gives you perk points when you rank up, and experience is generated by doing magical witchy stuff like casting spells, dueling, creating or drinking potions.
Maxis have stated directly that it's possible to learn every perk if you are devoted enough, as you can overmax Spellcaster.