• find: Finds by primary key, raises ActiveRecord::RecordNotFound if not found
• find_by: Finds first record matching conditions, returns nil if not found
• where: Returns ActiveRecord::Relation with all matching records

User.find(1)                    # Raises error if not found
User.find_by(email: "a@b.com")  # Returns nil if not found
User.where(active: true)        # Returns Relation (chainable)