Tagging is a collaborative, lightweight meta-data based technique for categorization of information. The basic concept of tagging is that team members assign tags as meta-data annotations to informational artifacts like web site addresses, wiki pages, shared files or tasks.

These tags can be seen as categories, but unlike classical categories there is no fixed vocabulary of tags the team members can choose from and tags are not hierarchical. As a result, a web page might be tagged more than one time with very similar tags that denote the same concept, e.g. "opensource", "open source", "open_source" and "oss".

Most tagging systems provide users visible feedback about what tags are already popular for a specific informational artifact or for a specific user group, so that users can in doubt decide for the most "powerful" tag for a certain item.

The power of tagging is that it provides a meta-data scheme that goes beyond simple full-text indexing, allows more than one view of information organization unlike classical directory structures and has a very low barrier of entry unlike classical meta-data-based approaches found in many knowledge bases.