DNS stands for Domain Name Service or Domain Name System. It can also refer to one or more of the Domain Name Servers that make up the system. The Domain Name System is a huge distributed database that performs the important task of translating domain names into I.P. addresses

Domain Names vs. IP addresses
Every computer on the Internet has a unique I.P. address. For instance, the computer that currently hosts cpwebhosting.net has an I.P. address of 67.18.163.98. This type of address is perfectly suited for computers contacting other computers, but people don't like to deal with long strings of numbers. Imagine if every web site you used on a regular basis had a 4 to 12 digit number you had to remember to access it! To solve this problem, memorable and/or guessable domain names are usually used to designate the computers that host web pages. Another advantage of domain names is that, unlike IP addresses, many domains can be hosted on one server. If that weren't possible there would need to be a lot more servers in the world and reseller web hosting would be difficult at best. Using domains, however. creates a new problem; unlike I.P. addresses, domain names contain no information about the computer's actual location on the Internet. So anytime someone requests a web page from the Internet the I.P. address is still needed. This is where the DNS comes in to play.

The Steps of a DNS Request
When someone tries to access your website with its domain name a request is sent to a local DNS server which is usually supplied by the ISP. This DNS server (and any other) can respond in one of two ways: if it already has the requested information, it transmits the IP address, if not it replies with the address of a DNS server that is more likely to have the information. The request now goes to an authoritative server for the top level domain. This server looks to find what name server is responsible for the second level domain. The domain registrar will typically maintain this server. At this point, the name servers that registrants provide to their registrar are found and sent.

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Some other DNS Terms are:-

Caching
This is obviously a lot of steps to go through and there are a huge number of requests happening every second. In order to reduce a load on the system, it's designers allowed for caching of DNS information. When a local DNS server finally finds the IP for a domain it stores it in a cache. How long this cache is used varies from location to location. There is also frequently caching at the browser and/or operating system levels. While caching makes the process much quicker and easier, it means that the information may not still be correct.

Propagation
The period of time after completing DNS changes with the registrar and before the DNS servers have quit using the old information is called Propagation. This is the main reason that changing servers can be such a headache for web hosts. The administrators of a DNS server can set the cache's time to live (or TTL) and there is no set standard. Although the authoritative servers are usually updated more frequently, they can also provide another level of delay. Another factor is how recently you or someone nearby has requested the site. All of these factors mean that there is no way to know how long propagation will take, and it will happen sooner for some users than others. A worst-case scenario is 48 hours, but many administrators are starting to use shorter TTLs, and our experience is that most users will usually have to wait 3 to 18 hours. If you are waiting for a domain to resolve you may also want to flush the DNS cache on your local computer to make sure that that is not out of date.