Technical
DNS
Also called Domain Name System.
DNS is the distributed lookup system that turns a hostname like `example.com` into the IP address a browser connects to. Every web request starts with a lookup, whether it is answered from cache or fetched fresh.
How it is measured
`dig example.com` shows the answer, the query time in milliseconds, and the server that replied. A cold lookup walks root, TLD, then the authoritative nameserver. A warm one returns from a resolver cache in a few milliseconds. Browser DevTools shows the same cost as the DNS bar in the timing breakdown.
Monitor from several resolvers (your ISP's, 8.8.8.8, 1.1.1.1) and from the authoritative server. Check that the answer is correct, not just fast: the right IP, from all of them, within the TTL.
Worked example
A consulting firm's site goes dark at 09:12 although the server is up. `dig` against the authoritative nameserver returns the right IP, but 8.8.8.8 returns SERVFAIL. A DS record at the registrar was left behind after a DNSSEC key rollover, so validating resolvers reject the answers.
Removing the DS record clears the failure on Google's resolver after about 20 minutes. The ISP resolver cached the failure and needs another 40.
How it differs
DNS is the whole system of resolvers, nameservers, and records. An A record is one entry in it, mapping a name to an IPv4 address. DNS is not limited to the web; email routing and domain verification use it too.
Common errors
Blaming the server when the name does not resolve. Testing only from your own machine's cache. Forgetting that the registrar's nameservers must match the host that holds the zone. Letting the domain expire. Setting a TTL of 86400 and then needing a fast change. Editing records in two places.
In practice
Write down where your authoritative nameservers are, who holds the login, and when the domain renews. Add an uptime check that resolves the name from outside your network. Keep an exported copy of the zone file.