Black hat SEO:
This is a technique used by website/blog owners to increase their pagerank..but this is often temporary and not preferred as this is compared with a spamming technique.
It just aims in atrracting the crawling methods of google spider.Search engine avoids those sites with use this technique.
Techniques like keyword stuffing(just including the compony/service name in possible places without any definite meaning),unnatural backlinks(creating links by paying money/illegal method)
It only focuses on ranking and not on the end user...hence it is often like a mirage wherein if people access the site they will soon have an idea that the site is spam.
White Hat SEO:
This is a legal way and accepted by search engines to increase a site's page rank.This includes usage of suggested keywords,topics,enriched content which drives traffic in a proper way.
Check Pushon.co.uk for few examples of black and white hat SEO techniques.It is always better that we do not follow the unethical practises.I am using the SEV service from Tucktail.com which covers the major SEO techniques and helps in obtaining the desired pagerank.They cover many techniques like keyword and suggested topics,submit the site to over 100 search engines,submission to directories,sitemap creation and also provide reports which will eventually improve the pagerank and website traffic in an ethical way.