Oracle

In the '90s, Magic's popularity created a slew of competitors. Many of these Collectible Card Games (CCGs) are 'dead,' meaning they are no longer produced and no new expansions will ever be released. The existing cards are still usable, of course- if you can find another player of a given game- but a big element of this genre is keeping things fresh with new releases. Magic, of course, is alive and well as it creeps towards the 30th anniversary of its beginning. But Magic is 'alive' in more senses than one- not only are new expansions coming out at a regular clip, but the wording on the cards is subject to change.

To play Magic, of course you must understand the basic rules and turn structure. But much of Magic is explained on the cards themselves- each card can break the rules (remember the golden rule, on the rules page). But when you're as old as Magic, it stands to reason that the designers change things over time, which includes the card wording. Changes happen for two main reasons:

1. To improve the game. Every now and then, Wizards looks at the state of the game and identifies areas of confusion. In response, they'll making wording or rules changes to make the game verbiage more intuitive, thematic, or streamlined. This article from 2009 highlights big changes to wording (and rules) ushered in with the release of Magic 2010.
2. To correct mistakes on the cards. This doesn't happen often (mercifully), but sometimes they can (for example) just misprint a card, or change their mind about what tribe a given creature belongs to. 

When a change is needed, what do you do? You can't recall the cards or print replacements- that would be logistically impossible to manage. Enter Oracle and Gatherer.

We begin with section 108.1 of the comprehensive rules:

108.1. Use the Oracle card reference when determining a card’s wording. A card’s Oracle text can be found using the Gatherer card database at Gatherer.Wizards.com.

To be absolutely certain that a given card's wording is accurate, you must consult this resource.  Let's look at an example.

Llanowar Elves is a card that's been printed many times over Magic's history. Consulting Gatherer, I pulled two copies of the card; one from Alpha (the first set ever), and one from Dominaria (released in 2018).  Note that the Oracle text for the game-related text is the same (the flavor text may differ, but that has no bearing on gameplay). Though the core of the card is the same- the power, toughness, and ability- there are a few differences:
- The presentation of the ability has changed quite a bit, as Wizards has evolved their symbols and keywords over the years.
- The Alpha Elf is not listed as a creature (just as a "summon"), nor as a druid- just an elf. The Dominaria Elf is clearly printed as a creature- Elf Druid.

Looking at the Oracle wording, Llanowar Elves is an Elf Druid. So any cards or abilities that affect Druids will affect the Elf- even the Alpha version, which doesn't have 'Druid' printed on it.

Another example: changing the tribe. The Homelands expansion had five creature cards where the art was clearly a minotaur, but only three actually say minotaur on them (the other two are 'ghost' and 'bodyguard'). Players noticed that immediately. Wizards corrected it- not on the card, but in Oracle:
- Anaba Ancestor is printed as a "ghost," but the Oracle text says it's a "minotaur spirit."
- Anaba Bodyguard is printed as "bodyguard," but the Oracle text says it's a "minotaur."

Sometimes the tribe names can change, too. If you look at the comprehensive rules from April 2020, section 205.3m lists all creature types and subtypes. It lists "Hound" but not "Dog."  Now look at the comprehensive rules from August 2020. That same section now lists "Dog" but not "Hound." What happened?  In July 2020, Core 2021 came out. It had a few cards that were given the Dog creature type.  Wizards decided that "Dog" and "Hound" were similar enough that they discontinued the latter in favor of the former.  When you look at Oracle, many cards printed as "Hound" are now listed as "Dog."

The above examples focused on creature types, but there are many other examples where wording has changed over the years:

• "Comes into play" became "enters the battlefield." 
• "Remove from the game" became "exile." 
• Just a few months ago, "put [x] cards from the top of a library into that player's graveyard" became "mill [x] cards."
• More keywords (like vigilance, lifelink, or deathtouch; see the Rules page) were introduced.

Gatherer- the database run by Wizards- is the only official source here. But many good card search engines- like Scryfall- link to this official reference (in Scryfall, on a given card's page, click "View on Gatherer"- see screenshot below).

The bottom line: the Oracle text in the Gatherer database is the authoritative reference for all card wording, and older cards have the newer wording automatically applied to them. You won't need this 99% of the time, but it's an important nuance that may pop up from time to time.