A page is a whole Web page. It's best for static content.
A story is a generally short news item or feature. You can create a story and have it post to a specific page (or pages) using the taxonomy feature. For example if you have an event at a specific branch for Seniors you could have the event post on both the branch events page and the Seniors page.
A blog entry is similar to a story but is attached to a user. If you wish to have several different blogs you would create different user identities for yourself and log in under each to submit a blog entry.
The book content type is suited for creating structured, multi-page hypertexts such as site resource guides, manuals, and Frequently Asked Questions (FAQs). It permits a document to have chapters, sections, subsections, etc. (what you're reading is a 'book')
A newsletter is a publication that has 'issues' to be sent to subscribed e-mail addresses.
A poll is a simple multiple-choice questionnaire, which displays the cumulative results of the answers to the poll.
The forum module lets you create threaded discussion forums for a particular topic on.
An event adds a new event to the events calendar. This can include recurring events, and signups.