I've noticed that some websites get indexed by Google really quickly, but not others. For example, on a forum website that I use, Google appears to index new posts within hours. For other websites, it can take days, or even weeks for new content to get indexed. Why is this?
It's true that some websites are indexed by Google far more frequently than others. In some cases, not just daily, but several times a day.
Some experts will tell you this is because of the number of inbound links the site has. However, while this might be part of the answer, it's only a small part of the answer. The main reason it happens is because Google has become aware of how often the content on the website is being updated.
Googlebot, the Google crawler mechanism, is always hungry for new content. Not just any old content, but good quality, unique, new content. If it discovers a source which is continually being updated with good quality new content, then Googlebot will revisit the site looking for the new content more often than it will visit a site that doesn't offer new content particularly often.
This doesn't mean that you should immediately install a forum module into your website and let your website visitors go wild posting anything they like. If you do operate a forum, you must be vigilant in moderating the posts from the users of the forum to ensure the quality of the new content being generated.
The key to success on Google, as always, comes down to offering top quality unique content, and lots of it. The more new content you add, the more often Google will visit your website to index it. Similarly, the more likely you are to have repeat website visitors, because your website will always be fresh and offering more and more interesting content for your it's visitors.