How DNS works
DNS is the internet's phone book. Your computer asks a resolver for the address of a name, and the resolver works down a chain of servers until one knows the answer, then remembers it for a while.
The chain of servers
The resolver, usually run by your internet provider or a public service, starts at a root server, which only says where the servers for the ending (com, org, ro) are. Those point to the authoritative server for the domain, which holds the real records.
Caching and TTL
Each answer carries a time to live (TTL). Resolvers and browsers keep the answer until it expires, so most lookups never reach the chain. This is also why a DNS change can take a while to show everywhere.
Records you meet
An A record maps a name to an IPv4 address and AAAA to IPv6. CNAME points a name at another name, MX says where mail goes and TXT holds text used for verification and email security.
Questions
Why does a DNS change take time?
Resolvers cache the old answer until its TTL runs out. Lower the TTL before a planned change.
What is a resolver?
The server that does the lookup for you and caches the result, usually your provider's or a public one.
A record or CNAME?
Use an A record for an address you control. Use a CNAME to point a name at another name, which a service gives you.
Is DNS encrypted?
Traditionally not. DNS over HTTPS and DNS over TLS encrypt the question between you and the resolver.