DNS is an acronym for Domain Name System or Domain Name Server. The Domain Name System assigns domain names to IP addresses. It allows humans to use simple domain names to load websites instead of having to remember cryptic IP addresses.
A record maps hostnames to the IP address of the host.
For example, the A record for exacthosting.com points the domain exacthosting.com at 220.127.116.11:
CNAME (Canonical Name) record points one name to another. CNAME are sometimes referred to as an alias. For example, mail.exacthosting.com is a CNAME pointed at mail.exacthosting.com.cust.hostedemail.com.
DNS caching: DNS caching is the length of a time that a DNS server will retain existing DNS records before checking for updates. The length of time that records are cached is usually determined by the TTL (see definition below) set by the authoritative nameserver, but some Internet Service Providers may have their DNS servers set to cache records for a longer period of time. This delay in updating DNS cache is commonly called DNS propagation.
DNS propagation: Depending on who you ask, you may receive different definitions for DNS propagation. This term is generally used to describe the time taken for the cache of TLD nameservers to be updated after changes are made to a domain name's authoritative nameservers. For example, after changing your domain's nameservers in exacthosting, the TLD nameserver may still direct queries to the old authoritative nameservers until it's cache has expired. DNS propagation is also used sometimes to describe the time taken for Internet Service Providers to update their own local DNS caches, since some Internet Service Providers do not always update their local cache based on the TTL of authoritative nameservers.
IP Address: Internet Protocol (IP) address. This is a numerical address assigned to computers and servers on the Internet. It's like a civic address for a computer. Computers need to have an IP address in order to find and communicate with each other over the Internet.
Hostname: A name or label that has been assigned to a host computer. exacthosting.com by itself is just a domain name, but after exacthosting.com has been pointed at an IP address (the host) by a DNS server, it becomes a hostname.
Hosts file: The hosts file is a text file in your computer's operating system that you can use to override external DNS records. It is similar to a zone file, but it is local to your computer system. You can edit your hosts file to force your computer to use a specific IP address for a domain name. Editing your hosts file should be done with extreme caution. If you change your hosts file to point www.exacthosting.com at 18.104.22.168, for example, then your computer will ALWAYS use 22.214.171.124 for www.exacthosting.com. This will prevent you from being able to load a website if the server IP address changes in the future, until you remove or edit the entry in the hosts file.
MX (Mail Exchange) record: A MX record specifies where email should be delivered. exacthosting's MX record is mx.exacthosting.com.cust.hostedemail.com. An MX record also contains a numeric priority. The lower the priority number, the higher the priority (yes, it sounds a little backwards). Some mail systems have different MX records for different inbound servers. If the first server is busy, the email gets routed to the next available server.
Nameservers (aka DNS servers): Nameservers, or DNS servers, are the computer systems that use the Domain Name System to translate hostnames into IP addresses that can be used by computers to communicate with each other over a network or the Internet. The fully qualified domain name, www.exacthosting.com, is not recognizable to a computer system until it queries the nameserver and finds out that the IP address for www.exacthosting.com is 126.96.36.199. Computers locate each other using numbers, while human beings are better at processing and understanding language. Domain names registered in exacthosting are pointed at our nameservers by default, ns1.exacthosting.com and ns2.exacthosting.com.
TTL (Time to Live): The TTL is a time, in seconds, set by an authoritative nameserver which tells non-authoritative servers how long to cache records. The exacthosting DNS server has a minimum TTL of 900 seconds. This means that other DNS servers are instructed not to query our nameserver more frequently than every 900 seconds (15 minutes) for new DNS updates.
Zone File: The zone file is a text file that contains a set of DNS records for the domain name. Each domain name that is registered in exacthosting is pointed at ns1.exacthosting.com and ns2.exacthosting.com. These are the exacthosting nameservers. These nameservers contain a zone file for every domain name registered in exacthosting. You can edit your zone file using the Manage DNS section under the Account tab when logged into your exacthosting account.