DNS
Also known as: Domain Name System, Nameservers
DNS is the system that translates a human-readable domain name into the numeric address of the server holding the website.
In full
The Domain Name System translates domain names people can remember into the IP addresses machines actually use to connect. When a browser is given a domain, it queries DNS to find where to send the request. Nameservers are the authoritative servers holding a domain's records and determining which service answers for it. Common record types include A records pointing to an IPv4 address, CNAME records aliasing one name to another, MX records directing email, and TXT records used for verification and email authentication. Changes propagate over minutes to hours depending on cached time-to-live values.
Why it matters
DNS controls both your website and your email. Whoever holds the domain and its nameservers holds effective control of both, which is why the domain should always be registered in the business's own name.