Relevant Content is one of MANY contributed modules (see a comparison here) which attempts to provide a list of content a reader might be interested in. These related/relevant/similar content modules achieve a similar goal using very different methods.. Some (such as Similar) use MySQL to do content searching whereas Relevant Content uses Taxonomy to produce a "score" based on how many terms two compared nodes share. For example, you could be reading a node tagged A, B, C. If Relevant Content compared that node to another tagged B, C, D then it would assign a score of 2 as the terms B and C are shared.
Relevant Content provides output in 2 ways. The first is the traditional Block. You can define as many block as you like, each one containing search settings such as node type filtering and vocabulary filtering along with result set limits. Each block can then be assigned to a region and display can be controlled using the core block display settings (eg, only display for certain roles, or on specific pages (inclusive or exclusive)). The second, and new, way is the CCK Field. The field, once added, is read only (ie, when you add or edit a node there are no settings added to the form) and is configured in much the same way as a block. Below is a screenshot with the two field types highlighted in their positions.