This article discusses three core modules - Book, Forum and Aggregator. The only thing they have in common is that they are very easy to understand and use, so in most cases the official documentation is sufficient for understanding how to use them.
Drupal 7 ships with two content types enabled - Article and Basic Page. You can add your own custom content types. Some modules will also come with pre-built content types. The most obvious example of a core module that comes with its own content type is Blog - once it is enabled, users with sufficient permissions can create Blog posts.
There are two more core modules that come with their content types - Forum and Book. Before we look at them it is important to point out that in Drupal each content type comes with its own set of permissions. For example, you may allow authenticated users to publish and edit Blog posts, but only administrators will have the right to create Books and book pages.
Official Drupal Documentation: http://drupal.org/documentation/modules/book
Books let you organize snippets of text into nested hierarchies with up and down and next / previous navigation. This is probably the most complicated statement you could make about Books; they are very simple to use and the official documentation explains how to set them up very well.
Just one more comment to make here: Book pages are the only Drupal nodes that come with next / previous buttons. Often you wish for the same functionality on other nodes, especially images. To achieve that, you must hack core (=inject PHP code, advanced users only), or add contributed modules (the recommended solution). As of now, several options exist; I recommend Flippy (http://drupal.org/project/flippy). Custom Pagers (http://drupal.org/project/custom_pagers) was a good solution in Drupal 6 days, but it hasn't been ported to Drupal 7 yet, although work is under way.
Official Drupal Documentation: http://drupal.org/node/796472
Forums are even easier than Books, all that you need to know about forums is in the official documentation. They provide discussion board functionality and can be nested and organized into groups. Most of the functionality is ready out of the box with little need for customization; but it is worth noting that since Forum Topics are a content type, they can be customized if needed (by adding or subtracting fields, modifying display rules, etc).
Official Drupal Documentation: http://drupal.org/documentation/modules/aggregator
The aggregator core module lets you bring in external feeds (such as RSS) and display them on your site as pages or as blocks. Aggregators are particularly useful if you run several related sites and want to display content on one site on another site (for example, a feed with departmental news on the college site). Once the external information is brought in, it is saved in the database and can be accessed just like any other local content (i.e. it can be used in Views to build custom displays of your feeds, etc).