Your web browser such as, Chrome or Internet Explorer, uses a thing called local cache. This means that when you visit a website more than once it doesn't always have to download everything, because it has local copies of some files like images or style sheet, thus making it quicker to load pages.
In theory your browser is supposed to figure out when a file has changed and download it again, the only problem is that, computers being computers, it doesn't always get it right.
Sometimes you might need to force your machine to update this information by clearing your cache. The specifics of this will depend which browser and operating system you're using. You can follow these steps to clear your cache.