With Search Engine Optimisation (SEO), as with many things, there are good ways and bad ways to approach and implement your campaign. SEO defines these as White Hat and Black Hat for the good and bad approaches respectively.
However, When it comes to SEO, it is imperative you are as ethical as possible in your approach. Obviously, the main aim you have is to rank as highly as possible, but you need to achieve this in a way which is accepted by the search engines and is also sustainable for your website. Black Hat SEO techniques attempt to exploit the search engine rankings using methods which are frowned upon such as keyword stuffing, illegal link building and spam.
All search engines have differing standards as to what are considered acceptable techniques. As such, there is no concrete definition for "Black Hat SEO" and it is therefore, to an extent, open to interpretation. Some search engines will insist that your content is of a benefit to the viewer and others suggest that any site which has an unnaturally high Search Engine Results Page (SERP) ranking is a culprit. Each search engine has its own algorithm for organising the rankings and also its own list of criteria for what it considers to be "Black Hat SEO".
The initial promise and results of "Black Hat SEO" can appear both tempting and promising by rocketing a site up the rankings. But, "Black Hat SEO" is generally considered a short term solution, as once the search engines become wise to a site's use of these techniques, they will penalise the site indefinitely. And, the search engines will find out!
Should you decide to employ a third party to take care of your SEO, make sure you research them thoroughly and don't be afraid to ask about the techniques they use. Many companies offer unrealistic promises which guarantee your site on the first page of google within 28 days, but don't be fooled. They either can't or they're using "Black Hat SEO" and your site will, sooner or later, be found out and penalised.