Drawing is the primary way of getting cards from your Spellbook or Atlas into your hand. It means to take a card from one of your decks and put it into your hand, resolving anything which may trigger from "drawing." Effects which say to put a card into your hand (e.g. ) do not trigger "draw" effects.
Whenever you are instructed to draw a "card," you may choose to draw a spell from your spellbook or a site from your atlas. When you are instructed to draw a "spell," you must draw from your Spellbook, and when instructed to draw a "site" you must draw from your Atlas.
If you are instructed to draw multiple "cards" by a single effect (e.g. ), you must declare how many sites and how many spells you intend to draw before drawing any of them.
Whenever you draw multiple cards, sites, or spells, you draw them one at a time and resolve any triggers after each individual card drawn (e.g. ).
If you draw a card but it ends up in a different zone, e.g. , Omphalos, etc., it still counts as you (the player) drawing it.