When a browser needs to resolve a domain name, it first checks the browser's DNS cache. If the address is not cached, the operating system's DNS cache is checked. If the OS does not have the answer, the request is sent to a recursive DNS resolver, often provided by the ISP.
If the recursive resolver does not have the answer cached, it queries the DNS hierarchy: the root DNS servers, then the appropriate TLD server (such as .com), and finally the domain's authoritative nameserver. The authoritative nameserver provides the DNS record, such as the IP address.
Caching at the browser, OS, and recursive DNS resolver reduces the number of DNS queries that need to travel through this hierarchy. This makes DNS resolution faster and reduces network traffic.
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