Every domain has at least two nameservers assigned to it and those nameservers hold records for the domain. However, when another server somewhere else in the world, queries the nameservers for DNS records, it will usually keep a local copy of the DNS record so that next time it needs to know what IP address the domain resolves to, it already has the answer. This is called "caching" and, for example, ISP's use this to speed up their network response times.
Once changes are made to the actual DNS records on the two nameservers that are assigned to your domain, these changes will gradually filter through to the other servers in the world so that everything is up to date, and it is this process that normally takes 24-48 hours.
