In a nutshell, the DNS handles translating human-readable domain names into the numeric IP addresses used by computers and Internet routers. The data is structured in a hierarchical manner and served from 13 clusters of root name servers that services the entire public Internet.
The most important fact to remember is that while DNS information can be cached for varying amounts of time within this hierarchy, only the DNS server serving your domain – known as the authoritative name server, holds the requisite IP address information. In simplified terms, knocking the authoritative name server out of action will disrupt the ability to resolve the IP address.
An inability to resolve an IP address will cause Internet software to fail. And because server-to-server communication also works on the same principle, the affected domain is rendered inaccessible, including the capability to send and receive emails. This is why Internet service providers and hosting companies routinely deploy multiple DNS servers.