CNAME
An alias: this name is that name. People sometimes say "C record"; they mean CNAME. There is no DNS type called a C record.
A CNAME says this owner name is an alias of another name. Resolvers then look up the target. Useful for pointing www at a CDN or a vendor hostname you do not control.
CNAME cannot sit next to other types at the same owner name, except DNSSEC-related records. Alias-to-alias chains add lookups and loops. MX and NS are not allowed to be CNAMEs. Keep chains short. The zone apex is usually A/AAAA, not a true CNAME.
See also: A record, AAAA record, MX record, DNS.