Keyword density:

The number of times a particular keyword appears on the web page. More times a particular word appears on the web page, more weight is given to that word by the search engine when that word matches a keyword searched.

Keyword stuffing:

Placing excessive amounts of keywords into the page copy and the HTML in such a way that it detracts from the readability and usability of a given page for the purpose of boosting the page's rankings in the search engines. This includes hiding keywords on the page by making the text the same color as the background, hiding keywords in comment tags, overfilling alt tags with long strings of keywords, etc. Keyword stuffing is just another shady way of gaming with search engines and, as such, its use should be strongly discouraged.

Thanks and Regards
Richard Smith

Online Reputation Management