Are you familiar with the term Gray Hat SEO? Simply put, this refers to the act of performing search engine optimization on your website in a way that simply skirts the boundary between legitimate and illegitimate SEO tactics. If you are using these techniques, you are still in the safe zone. However, all of your actions may suddenly become illegal the next time Google decides to update its search engine guidelines.
Not quite getting it, are we? Well, the following example should help. Let us suppose you have a website selling liquor online. Obviously, you need to verify whether visitors are above 21 years of age, before you can let them access your website. Say, you use a pop up window to verify the age of any visitor. However, you do not want to let crawlers from Google to access the pop up, since it would restrict links from your website to be visible on search engine result pages. So, you manipulate the robots.txt to ensure that Google spiders can only crawl through the main page of the site, without having any access to the page with the pop ups.
The example given above is obviously not a clear example of white hat SEO, since it involves cloaking of a link in order to get more web traffic. On the other hand, it is not exactly black hat SEO either, since you do not have any malicious intent. Such are gray hat SEO tactics – which do not break any laws yet, but may very well do so at any given moment. You would do well to remain away from such tactics!