Caching is used widely across the Internet to cut down the amount of time it takes to request information. It works by keeping a temporary copy of information that has been requested locally for a defined amount of time.
The most common forms of caching web pages occur with your web browser and with your Internet Service Provider (ISP, such as Verizon, AOL, Earthlink, etc.). Most web browsers cache pages when you visit a web site so that the next time you go to that page it appears to load faster, because it is using a local copy rather than requesting a new copy be sent across the Internet.
Some people know about the browser caching, but still get taken aback by ISP caching. ISP caching works in much the same way as browser caching. Once you have visited a website your ISP may cache those pages so that they appear to load faster the next time you visit them. The main problem with this is that unlike your browser cache you can not delete these temporary files, instead you have to wait until your ISPs cache expires and it requests fresh copies of the files. This can be very frustrating if you are trying to develop or make changes to your website -- or even to look at new information.
Unfortunately there is very little that can be done about ISP caching except to change your ISP. Most ISPs do not cache pages for more than half an hour or so but there are some that take considerably longer. AOL and Compuserve are the ISPs that seem to keep them for a very long time -- sometimes even more than 24 hours, though they claim they do not.
If you are getting unusual results and suspect that caching may be your problem, first hope that problem is simple, and clear your browser's cache. (If for some reason you don't want to do that and you have multiple browsers on your computer, you can just go to the page with another browser).