These cards increase the amount of permanents on the board or provide some advantage that makes your cards better and clearly advances your board. Because you spend a card to get access to more cards, these are considered to be card advantage.
Tangible-
When you play a card, you wind up down a card in hand. Resolving a card with Tangible advantage results in you having more pieces of cardboard in your hand or on the battlefield than you had when the turn began. One way to do this is with Engine cards, like Setessan Champion’s Constellation ability, which draws cards whenever a condition is met. Another way to do this is with spells that draw multiple cards, like Divination.
Intangible-
Especially in multiplayer games, there are plenty of advantages to be gained besides just having more pieces of cardboard. Resolving a card with Intangible advantage results in some benefit to all or some players, but doesn’t put you ahead. You might not have more cards than other players, but you have something they don’t: leverage. One way to do this is with mutually-beneficial cards like Rousing of Souls’s parlay ability, which draws everyone cards. Another way to do this is with spells that draw everyone cards, like Howling Mine.
Cantrip-
Walking a sort of middle ground between advantage and disadvantage are cards that provide effects that you want otherwise, but happen to also have the phrase “draw a card” stapled on. If you want to be able to draw cards, but don’t want to devote specific slots to it, this is a solid way to do both. One way to do this is to find spells like Root Out that have Investigate, to make a Clue you can crack later to draw a card. Another way is to look for cards like Crimson Wisps that can work with engine-based commanders like Feather, the Redeemed to create Tangible engines.