What exactly is keyword stuffing and why should you avoid using it on your website?

Keyword Stuffing is using your keywords too often on your web page. It is quite literally "stuffing" the keyword in as often as you can in the hope that your page will rank high with the search engines. It's a classic Black Hat SEO trick that continues to get websites banned from Google and any of the other search engines that take a dim view of this type of trick. Of course like all of the Black Hat techniques it did work at one time and that is why some many people used it. These days most of the search engines are wise to is and frequently rank pages low that are seen to be using it.

However a lot of people end up stuffing quite innocently. When you are concentrating on writing a page to promote a keyword it becomes very easy to subconsciously keep using the keyword. Before you know where you are, you have crossed the line and started to enter into keyword stuffing territory.

So where is the dividing line between good keyword density and stuffing?

The search engines all keep this threshold to themselves as they know that if we knew at what point a keyword is overused then we would all write our pages up to that maximum and no more.
So if the search engines will not tell us where the limit is, how do we know where to draw the line?

The trick is to let the search engines show you what they consider to be good keyword density and to do this you simply type in your keyword and then take a look at the top four or five sites that they list. Check how often those pages use the keyword phrase and also pay attention to where they are using it.

In my experience most good sites will have the keywords mentioned once in the title tag, once in the description meta tag, once in one of the heading tags and then they will mention it three or four times through the copy on the page.

Of course three or four might be too many times if the page has very little text. Some experts quote that a keyword can be used safely every 400 to 500 words. That seems reasonable from some of the sites I've seen. However you should check some pages for yourself and get used to the keyword density that is being used on the high ranking sites.