Having a site ranked highly in the search results is extremely profitable for many companies. Hence, Google and the other search engines are continually battling to keep their results pages as relevant as possible, and to reduce the ability of webmasters to artificially influence the rankings.

Google publishes specific quality guidelines for webmasters which it is strongly recommended you read.

Search engine optimisation can be a quite tedious and monotonous task, especially when it comes to seeking out incoming links for your site. People new to SEO are often very susceptible to various schemes that are promoted across the internet that will suposed reduce the workload, or dream up ideas themselves that they think will get them to the top of the rankings more quickly.

One such common "idea" is placing many repitions of a key phrase in hidden text on a page so that it's visible to search engines, but not website visitors. There are several ways of doing this e.g. while coloured text on a white background.

Such actions are specifically against Google's Quality Guidelines:
Avoid tricks intended to improve search engine rankings. A good rule of thumb is whether you'd feel comfortable explaining what you've done to a website that competes with you. Another useful test is to ask, "Does this help my users? Would I do this if search engines didn't exist?"

Another are link schemes such as link farms. This is a technique where you agree to place hundreds of links to all manner of unrelated sites on a links page on your site in exchange for those sites doing the same and thereby creating many incoming links to your site. Often this is carried out by an automated process. It is considered Search Engine Spamming and it is against the Terms of Service of most Search Engines to be involved with these linking schemes.

Google's Quality Guidelines again warn against this practice:
Don't participate in link schemes designed to increase your site's ranking or PageRank. In particular, avoid links to web spammers or "bad neighborhoods" on the web, as your own ranking may be affected adversely by those links.

These are just a couple of examples to illustrate the point. Be warned - Google is particularly sophisticated at discovering and weeding out spam sites from it's search engine results. The advice is to follow Google's guidelines else you run a high probability of being penalised or possibly removed completely from Google's results.