Avoid Hidden Text – Playing Hide and Seek With Visitors:

This technique is most commonly used by bloggers where they select a bunch of keywords for which they want to get ranked high in SERP and they place keywords on the pages in such away that are only readable by Bots, not   for human beings. They usually use a hide text with DIV tag where they stuff all these keywords, by blending text with same background color, hiding text behind images so that only crawlers can see them. ajmalseotips.blogspot.com