A guest post is an article which is written from a guest author. It’s published on a high quality blog or website.

In high quality guest posts you will often find the real name of the author mentioned at the end of the post, this leads to an increase of the AuthorRank. Low quality guest posts are often paid publications with cheap content.

The difference between link building with guest posts which is tolerated by Google and a spam of the same or similar articles on many pages is something which humans and computers do recognize.
Matt Cutts published a video about this subject

It can be very time consuming to publish a guest article, monitoring the publication afterwards and write comments on the reactions, but if it is published on a high quality platform then it is possible that you can get great results and links with this method. In additions guest posts have also a positive impact on the brand and the reputation of the author.