I play D&D 5e and sometimes come here with questions. But sometimes I see things referenced here as sources that I don't have or am not aware of....
Which are the "official rules" for D&D 5e, and where do I find them?
These three books form the official rulebooks for D&D5e. They are also sometimes known as the "core set" or "core books".1
The Basic Rules and SRD are both freely available subsets of the core set.1.5 They add no new rules to the core set and exist (Basic Rules) to provide a free channel to play and run games and (SRD) to support the development of third-party products.
D&D Beyond is WotC's official (
though no longer 3rd-party) online source for rules. It hosts paid access to electronic copies of the core books (PHB, DMG, MM) as well as the freely accessible Basic Rules + SRD. It also hosts paid access to all official supplemental sourcebooks (e.g. Xanathar's Guide to Everything) and adventure books (e.g. Rise of Tiamat).
However, do note that D&D Beyond also hosts recent Unearthed Arcana, homebrew, 3rd party and other content which are not official rules sources.
Wizards of the Coast has published a number of other books in 5e which fall outside of the core set. Some of these contain character options, new spells, and equipment not seen in the core set. In addition, some adventure paths featured supplemental "player's guides" which may contain new material. While not part of the "core game," these are "official" sources for rules pertaining to their contents.
Unearthed Arcana are articles published by Wizards of the Coast featuring new rules or options. However, they are explicitly experimental and not official. The following disclaimer, or something like it, appears in the head-matter of most UA articles:
You can think of the material presented in this series as similar to the first wave of the fifth edition playtest. These game mechanics are in draft form, usable in your campaign but not fully tempered by playtests and design iterations. They are highly volatile and might be unstable; if you use them, be ready to rule on any issues that come up. They’re written in pencil, not ink. For these reasons, material in this column is not legal in D&D Organized Play events.
Sage Advice (Twitter) and, really, the rest of Twitter
The public statements of the D&D team, or anyone else at Wizards of the Coast, are not official rulings; they are advice. One exception: the game’s rules manager, Jeremy Crawford (@JeremyECrawford), can make official rulings and usually does so in Sage Advice. (Sage Advice Compendium, emphases mine.)
So Jeremy's tweets might be official, and nobody else's are.
And now we find ourselves at the oft-cited Sage Advice twitter archive. This is an excellent resource maintained by an awesome volunteer that concatenates tweets on rules/rulings from a number of D&D developers. However the page itself tells us that it is "an unofficial site of Wizards of the Coast about Dungeons & Dragons."5 Additionally, the two Lead Designers often remind us that only rulings from Jeremy Crawford could be considered "official".6 So the material in Sage Advice (twitter) are simply statements of designer intent, with the possible exception of Jeremy Crawford's tweets carrying more weight.
The problem is, even Jeremy's tweets sometimes contradict RAW or Jeremy's other tweets. Compare two tweets on rage+heavy armor (1, 2), two tweets on cutting words (linked here), or a tweet on spirit guardians later reversed in proper SA (1, 2). There's no way for one reading even Jeremy's tweets to know whether they've (a) already been contradicted officially or (b) whether they already contradict settled rules. If Unearthed Arcana are the volatile precursors to new sourcebooks, then developer tweets--even by Jeremy--are the volatile precursors to Sage Advice articles.
To summarize: a tweet from Jeremy might be official, and no others could be. I have yet to see a tweet where Jeremy has said "this one's official" and have (personally) adopted a stance of being skeptical of all of them.
1 - It should be noted here that "if in doubt, the Monster Manual version of a creature's stat block is authoritative." (From the PHB errata) This matters in the case where a stat block appears in PHB appendix D and in the MM and they contradict.
1.5 - Interestingly, though, Jeremy Crawford has said that the SRD is not an official rules source. This comes into play on the rare occasions when the wording between SRD and corebooks isn't consistent.
2 - And it's a little trickier, because early PHB + errata isn't exactly equal to later PHB printings: some wordings aren't the same, some changes were made that aren't reflected in the errata, and at least one erratum (IIRC) isn't captured in a later printing. As a general rule, then, later printings should be held as "more" official when they conflict with earlier printings, even adjusting for errata. (With thanks to @timster for the reminder.)
3 - Through the editions Sage Advice columns have held differing stances on how "authoritative" they are in interpreting the published rules. In this edition Sage Advice rulings are held as official and authoritative, though not rising to the level of errata. (The Sage Advice Compendium contains an explanation of the distinction that is being drawn between an erratum and a clarification.)
4 - The options in Volo's and MToF was republished in MotM with revisions, marking the older books as legacy content (and thus no longer official). Some content from setting books was also republished in MotM. Some spells in EEPC were revised for inclusion into XGtE; the EEPC was re-released with those revisions included. If using the EEPC be sure you are using an updated version.
5 - Look near the bottom of the leftmost column. Note that this site is "of" Wizards of the Coast in the same fashion that my desk is "of" wood: the site's made of things WotC, not made by WotC.
6- Sage Advice (articles) tell us Jeremy's rulings as published in Sage Advice articles are official, and Mike Mearls himself often reminds readers that his rulings are not official but are just the rulings of one (very well-informed) GM. (Example here.)