Articles are written as an informational source about a product or service that can be useful to someone else. You can write articles about "How to rank a website in Google" or "How to build a great looking website". They need to be detailed enough for someone to read it and click the link to your website for more information.
There are a lot of people out there that, believe it or not, make websites and blogs for their own enjoyment. Those people copy and paste your article and post it to their blog if they think it's imformative and useful information to their readers. That gives your website a link. Link building is another form of SEO services to help the ranking of your website.