Mythos vs Non-Mythos Spells

There are many people capable of wielding magic on Earth-218. Only a comparatively small handful of these practitioners run the risk of being driven insane by the fundamental forces they shape. 

Why?

The short answer is the source of the powers they direct. "Standard" magic is an arcane energy created by all living things and can shaped and controlled by a trained mage who may be referred to as a Wizard, Sorcerer, Priest, Warlock, or any number of regional titles.

Note: "Warlock" is often also used to refer to any evil spellcaster who channels arcane power, regardless of the source of their magic. 

Note #2: Technically, Dr. Stephen Strange is a Warlock due to his drawing of power from the Vishanti even though he is referred to as the "Sorcerer Supreme". (His studies of other forms of magic most likely make him a multiclass Wizard/Sorcerer/Warlock).

Mythos magic on the other hand, specifically calls upon power channeled from beings called the Great Old Ones, Outer Gods, or Elder Gods. These alien entities surpass the capacity of mortal understanding by so much that merely being exposed to them can drive a human (or elf, dwarf, gnome, etc) insane. Beings of this magnitude include Cthulhu, Shuma-Gorath, Yokoth and Glythur, Nyarlathotep, and Gozer the Gozerian. 

These alien beings differ from the gods who grant magic to clerics, paladins, and other divine spellcasters who are essentially much more powerful versions of their worshippers who draw power from being worshipped (and can die or fade away without that adulation - something these Old Ones do not require).

Additionally, Mythos spells do not have levels and there is no limit to the number of times a spell can be cast except for the toll it takes physically, mentally, and emotionally, on the caster (via Sanity damage or Ability Drain). Additionally, anyone, at least in theory, could cast a Mythos spell as long as they had access to a tome or scroll containing the spell.