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What is DNS? (with the record types explained)
DNS is a lookup system with caching at every layer. Almost everything that confuses people about it — especially why a change you just made is not visible yet — comes from the caching, not the lookup.
The lookup
A browser needs an IP address to open a connection, and you typed a name. DNS is the system that turns one into the other. Your machine asks a resolver, usually run by your network or by a public provider. If the resolver does not already know the answer it walks down the tree: the root servers say who runs .com, the .com registry says which nameservers are authoritative for your domain — the delegation a WHOIS lookup reports back to you — and those nameservers give the final answer.
The authoritative nameservers hold the truth. Everything else holds a copy with an expiry date.
Why changes take time
Every record carries a TTL — the number of seconds a resolver may keep serving its cached copy without asking again. Change a record and the authoritative servers update almost immediately, but every resolver that already cached the old value keeps serving it until its TTL runs out.
That is why two people can see different things at the same moment. It is not a bug and there is nothing to fix; it resolves itself. If you know a change is coming — a move to a new host, or a domain transfer — lower the TTL to 300 a day beforehand, make the change, then raise it again once it has settled.
Browsers cache separately
Resolver caching is not the only layer. Chrome keeps its own internal host cache, independent of the operating system, and may use its own encrypted DNS provider as well. So a name can resolve correctly in one browser and incorrectly in another on the same machine at the same time.
Two things follow. First, before concluding that DNS is broken, query an authoritative nameserver directly — dig +short @ns1.example.net yourdomain.com A, or run a DNS lookup that lists every record with its TTL on the A and AAAA rows — and compare it with what the browser does. Second, if they disagree, the problem is local: clear the browser's host cache, or test in a private window. That local caching is also the usual reason a site looks broken straight after connecting a domain to a website, when the records themselves are already correct.
The records you will actually meet
Each type is covered in more detail in DNS records explained.
Propagation is a misleading word
Nothing is pushed anywhere. "Propagation" is just the old cached copies expiring one by one, at different times, all over the internet. Nobody can speed it up, and a registrar who offers to is selling you the passage of time.