SEO is art of combining many methods to get top listed in the search engine. Some method is good and safe, but some may be illegal. If Google finds a site containing this so called illegal SEO, it will be kicked out really fast. What shouldn’t we do in SEO:
Keyword Stuffing
This means putting “keyword” everywhere in the site repeatedly. Absolutely everywhere even in the META tags and image alt text
Hidden Text
Putting keyword text which not appear at the front end, but readable by search engine crawler. For example, putting white text containing keyword on white background.
Mirror Website
Having many website with many URL but contain exactly same content. Each website is heavily exchanging link one to another.
Link Farm
Build one page with hundreds of link in it to certain irrelevant webpage.
What exactly the risk of using illegal SEO? First, your site maybe dropped from the list. Or even worse, you’ll be banned from search engine index for lifetime. No more your name in Google.