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DNS records explained: A, AAAA, CNAME, MX, TXT
Every DNS record answers one question about a name. Five record types cover almost everything a website or a mailbox needs, and most DNS problems come from mixing them up.
A and AAAA
An A record maps a name to an IPv4 address, such as 76.76.21.21. AAAA does the same for IPv6. A name can carry several of each, and resolvers rotate between them. These are the records that make the bare domain load.
CNAME
A CNAME says "this name is an alias for that name". Pointing www at the bare domain is the classic use, and the usual way of handling www when you connect a domain to a website. Because it hands off the whole name, a CNAME cannot share a name with any other record, which is why you cannot put one on the apex. Hosts that need an alias at the apex offer ALIAS or ANAME records instead.
MX
MX records tell sending mail servers where to deliver. Each carries a priority; lower numbers are tried first. An MX must point at a hostname that has its own A or AAAA record, never at an IP address directly.
TXT
Free-form text, used for verification and email policy. SPF (v=spf1 …) lists who may send mail for the domain, and DMARC at _dmarc says what receivers should do when a message fails. Most site-verification flows ask for a TXT record at the apex, and when one keeps failing the first step is a DNS lookup to see the TXT records a resolver actually returns.
TTL
Every record carries a TTL: how many seconds resolvers may cache it, which is why a DNS change takes time to show up. Lower it to 300 a day before a planned change and raise it to 3600 or more once the change has settled.
Edits only take effect at the nameservers the registry actually lists for your domain, so when a change appears to do nothing at all, check which nameservers are on record with our WHOIS / RDAP lookup before editing the same zone again.