A tag is a keyword or a phrase associated with a post as a means of classification.  In each post, the tags appear as a list between the title and the text of the post. If you click any tag in such a list, all posts in iMechanica with the same tag will show up.  In effect, tags classify posts in iMech into channels.  

When you write a post, you have the option to assign the post with one of the featured tags, which are listed in the header:  research,  education, mechanicians, opinions, industries, conferences, jobs, and tips. 

You can also assign free tags of your own choice.  For example, you may choose thin film, plasticity, fracture, JMPS, Budiansky, or anything you like, or nothing at all. If you want to assign multiple tags, separating them with commas.