Search engine optimization or SEO is the process of gaining better search engine rankings through technical means. The process should focus on optimising of your website to give search engines better information to go on. There are many tricks and techniques involved in SEO. A big part of it is getting your website linked to from many other websites. That is where some blackhat methods like link spamming can become selfish, anti-social, and destructive. There are ethical or whitehat SEO methods like submitting your site to web directory services, optimizing your page code with valid keywords, and creating real partnerships with websites.
When hiring an SEO business to promote your website be careful you know what methods they are using. Some SEO businesses will use any method necessary to get their clients top rankings. You may not know exactly what methods they are using, and if they are link spamming, then those links are making you look bad. Spamming websites with irrelavent links, is a technique which should not be used by any reputable SEO business. It is selfish, anti-social, and destructive.
If you choose the wrong SEO company your site could end up being totally removed from Google for really dirty tricks. Blackhat methods may get quick results, but if banned from Google for them, it is an expensive mistake to fix and takes a long time.
If your SEO company is plastering spam links all over the web, it's your name they are doing it with. When people have to clear up a trail of destruction left by SEO link spamming, they are not going to think nice things about your company! Information on attacks from spammers spreads fast. You may well wind up being listed as a spammer. That's not the kind of publicity you were hoping for now is it?
Google provides a page with good information about SEO that also provides some hints on how to spot bad SEOs.
Of course, search engine optimization is not only about getting links from other sites. Search engine optimization should start on your own site. Provide good and useful content and others will start linking to you. Get rid of stupid flash intros and search engines will like you more. Provide your content as text and not as images. Provide good content!