When people talk about article spinners they are usually talking about a software that will take an original article and alter it. The software would modify the sentence structure and some of the words used in order to produce an unique piece of article. Most of these applications are downloaded to your PC while others are accessible online. The benefit of the online software packages is that you can access them from anywhere. Most individuals will use spinning software to generate article for distribution to other websites but will continue to write the content for their money sites physically.
Writing articles is an excellent SEO strategy but one that demands time and money. You can outsource this job to content writers but even if you employ cheaper labor the expenses will increase. A lot of people also find that the cheaper writers will not possess the English expertise necessary to produce readable articles.
The alternative is to apply software to rewrite the article but however this could take time depending on whether you are doing it at sentence or paragraph level.
With article spinners it is easy to click a button which identifies all the synonyms in the article. It could then replace these synonyms with alternatives from the built in thesaurus. Most of the better spinners have created a large thesaurus mostly from contributions made by various users. The idea is that the final article would actually use different words but maintain the same meaning.
The reality is that this isn't usually the case. It doesn't matter which article spinner you use, you should always evaluate the quality of the articles produced. Although some article directories are automated and thus their material is not verified by human reviewers, most of the better ones are. Most people will not permit a badly spun article for inclusion into their directory.