Google Panda is the codename given to a major update to the Google Algorithm, that was released in February 2011. The main focus of this update was to increase the rank of high quality websites (and reduce the rank of low quality or 'thin' websites) in order to encourage webmasters to improve the quality of their content.

Google have published a number of articles on their official blog providing guidance on building high quality sites.  The main aspects include:

1. Authority of the author;
2. Trustworthiness of the site;
3. Duplicate content (repeated content);
4. Quality of the publication;
5. Spelling, style errors;
6. Readability of the article;
7. Originality of the content;
8. Exclusivity of the article (to the site);
9. Comprehensiveness;
10. Bookmark or share value;
11. Attention to detail.

In short the change is all about providing good, original, high quality article on the website and removing any low quality articles as these can have a negative impact on the site.

"The Panda change tackles the difficult task of algorithmically assessing website quality"