An Analogy of DNS Resolution
Imagine that you own a store in the high street. Now, for people to find your store, they can’t just look it up, or walk down the street or anything. They have to rely on some guy (an “informer”) who stands outside their house and says, “The Widget Shop? Yes, it’s on the high street.” People will be able to get to your store, and they’ll remember what the informer told them, but only for a week or so. After that they’ll be like, “Where’s The Widget Shop again?” and have to ask the informer.
In order for the informers to even know where to send people, you have to let them know your address. But you can’t go and tell them all yourself, so you’ll usually just tell one (the one standing outside your own shop) and he’ll pass on the information to his informer friends. Some of the informers in newer streets and near big companies are really smart, and have phones so they can call each other and let each other know about new shop addresses, but other informers are really slow and have to wait for another informer to come and tell them.
Now, imagine that your shop moves. You’ll tell your informer in front of your shop that you’re leaving, but you can’t give him your new address, as you don’t have it yet. You have to wait until you get to the new address and tell the informer there. He’ll then start telling the other informers, but there are a lot of them and there’s also the other informer from the old street just saying “They’re not here any more.”
So basically, some people may have up-to-date information from their informer about the whereabouts of your new shop, but it will take a while before everyone does.