Arelith has a large amount of custom tweaks that help build on nearly every aspect of character creation. If you're new to Arelith it can be a bit overwhelming, but you don't need to learn it all right away. Nearly every class is playable as their base class without choosing any server specific modifications, although there are a few classes that have their base functionality changed slightly(See below).
See the specific page for more information regarding each category. The brief descriptions here are not even close to providing all the details, but should help you get started.
Classes to read about
While nearly every class has modifications that can you opt out of, these base classes have been altered and can't be played without the changes. If you plan on making a Fighter, Barbarian or any class that uses an Animal Companion or Familiar, it is recommended you read about their changes. There's also a number of Spell changes.
Backgrounds are an optional selection on character creation that adds subtle influences to help support the characters past before arriving on Arelith. Most commonly they add Saving Throw and Skill penalties and bonuses.
Gifts are bonuses to your character that make you level more slowly(see ECL). These bonuses include but are not limited to Skill Bonuses, Stat Bonuses, Resistances, and more.
Many races come with optional sub-races, such as Sun Elf, Moon Elf, Wood Elf, Wild Elf and Drow for a character who choose elf on creation. No Subrace is also a choice. Each subrace has varying bonuses, although you only get the bonuses if you take the Subrace Properties Gift. This means you can choose a subrace without any modification to your characters stats if you want.
Arelith has unique paths for many classes, although these can be quite complex and will take time to learn. Using these paths will allow many character designs you never previously thought possible. Examples of paths include Warlock, Favored Soul and much much more. Some paths alter the classes play style so dramatically that it is essentially an entirely new class. All paths for every are optional.
Familiars have been changed to be utilities to the mage. They specialize in certain skills and have unique abilities that compliment their style. All familiars do no damage in combat, but have been given Epic Dodge, Self Concealment and other feats to increase their survivability.
Animal Companions have been sectioned off into categories, each specializing in certain combat styles. The bear tanks, the spider has sneak attacks and crippling strike, etc. (see the page for full details)
Arelith allows you to remake a character and/or delevel under certain conditions, however you are never given XP back nor are you allowed to transfer levels nor items to the new character. You have to start from Square 1 again. See the article for exclusions.