Tagging is a way of letting other people knew about a web site or blog. In the case of a blog, you will usually see a portion on the page that invites you to tag the site with keywords these tags then lead other people with similar interests to the blog

On web sites you can bookmark and tag sites in much the same way. The two main tagging sites are del.icio.us and StumbleUpon, both of these enable you to make comments about the site and add keyword tags or phrases that help others to find it.