The distributed phone book that turns hostnames into IP addresses.
Plain English: computers route by numbers (IP addresses), but humans type names (google.com). DNS is the global lookup system that translates between them. Every site visit starts with a DNS query.
A hierarchical, distributed key-value store mapping human-readable names (api.example.com) to IP addresses. Lookups go through a chain: local cache → resolver → root → TLD (.com) → authoritative server.
Users type names, computers route to IPs. DNS bridges the gap, and does it at a scale where the entire internet's name lookups happen in milliseconds.
Your OS asks a resolver (often your ISP or 8.8.8.8). The resolver checks its cache; if miss, it walks the hierarchy. Result is cached for the TTL specified in the DNS record. Record types: A (IPv4), AAAA (IPv6), CNAME (alias), MX (mail), TXT (arbitrary, often for verification), NS (delegates to another nameserver).
A BGP misconfig wiped Facebook from the internet for 6 hours, including their badge access.
A routine command intended to assess global backbone capacity was issued, but a bug in Facebook's audit tool failed to stop it. The command withdrew all Facebook BGP routes, taking the company off the internet. Worse: the same DNS infrastructure that announced their existence to the world also gated their internal tools, including the badge-access system at the datacenters. Engineers couldn't VPN in, couldn't open the doors, couldn't even reach the management plane to roll back. Recovery required physically driving engineers to the datacenter floor. The lesson: never let your control plane depend on your data plane. Out-of-band access has to actually be out-of-band.