DNS (Domain Name System) translates your domain name (ex. civicplus.com) to an IP address, which tells the internet where your site is located. To illustrate, you can think of DNS like the 'phone book of the internet'. In a phone book, you need two things to get a phone call - a name and a number. Similarly, to be found on the web you need to have a domain name (name) and DNS records (number) to tell the traffic what servers to ask to find the site and how to get there. You can find more information about how DNS works in the article: How Domain Name Servers Work.
Common Types of DNS Records
IP Address (A Record)
An "Address Record" is used to map the domain name server (or hostname) to its numeric IP address. Simply put, the A record resolves a domain name or points the domain name to a specific location by means of the IP address.
Mail Servers (MX Record)
A "Mail Exchange Record" is used to create a mail route for a domain name. A domain name can have multiple mail routes, and each will be assigned a priority number or preference number. The mail route with the lowest number identifies the server responsible for the domain and any other mail servers listed will be used as backups.
Host Aliases (CNAME Record)
A CNAME or "Canonical Name" record is a DNS record that can be used to create an alias for a domain and allow traffic for the domain name to be redirected somewhere else.