Propagation is the projected length of time it takes for the DNS to update from a change made to any DNS records across the internet. While your DNS is propagating, some of your website visitors will not be able to view the new changes to your site until the propagation time has completed.
When does propagation happen?
Propagation takes place whenever a change is made in the DNS zone or to any nameservers. Once a change is made, propagation time will vary depending on the record that was modified. A nameserver change can take up to 48 hours to resolve, while a change in the DNS zone can take up to 8 hours.
The area your visitors are located will dictate how long it may take them to view the new changes that have been made. You might be able to view the changes made, but you neighbor may not be able to see the changes yet. Propagation usually depends on three different factors; your Internet Service Provider, the geographical location, and Time To Live (TTL) settings for your DNS records.
No hosting/service provider has control to expedite propagation, but there are sites you can use to view where the propagation has been completed. Here are are a couple links to DNS propagation checker tools:
DNS Caching
Every time you use a domain name on your local computer its DNS information will be stored in the cache. This can be cached information in your web browser, local cache on your computer’s operating system, DNS cache stored by your Internet service provider, content management system, etc.
DNS records are stored in cache mainly to improve the performance of DNS queries. Every DNS record has a TTL (Time to Live) value, which is the time DNS servers should store that record in the cache. Even if a record is changed, DNS servers will continue working with its formal value from cache until this time has passed. This, along with other factors all effect propagation and why the time frame may vary.
Most computers will cache DNS for 48 hours until it updates. If the DNS is caching in your computer, it may be possible to flush the DNS on your computer so that it can refresh and look up the IP address for the domain again. If your browser has cached any information, this can still cause you to see your old content, but browser caching has absolutely nothing to do with DNS. You can easily clear your browser cache to get a fresh copy from the server.