A single web site could contain many types of content, such as informational pages, news items, polls, blog posts, real estate listings, etc. In Drupal, each item of content is called a node, and each node belongs to a single content type, which defines various default settings for nodes of that type, such as whether the node is published automatically and whether comments are permitted. (Note that in previous versions of Drupal, content types were known as node types.)
When you first install Drupal with the default installation profile (in contrast to the minimal installation profile), you will have two content types defined: "Article" and "Basic page". When you enable other core and contributed modules (by visiting Modules), you will find that you have other content types available; you can also create your own content types (see below).
Content types in Drupal 7 and 8
The Article content type (formerly, "story") is enabled in Drupal in the default installation profile. Articles are generally used for information that is updated more frequently and often cross-referenced and categorized (such as news items or resources). By default, Articles are sorted with the most recent post at the top, but this can be customized with contributed modules like Views.
The Basic page content type is enabled in Drupal in the default installation profile. Typically Basic pages are used for static content that can (but are not required to) be linked into the main navigation bar. This is one of the most "basic" content types and can be very flexible.
The Blog (short for weblog) content type was removed in Drupal 8. It is an online journal or diary, and Blog module allows registered users on your site to create their own blogs. Each entry in a user blog has content type Blog Entry.
Book pages are designed to be part of a collaborative book, enabled by the core Book module. An example of a collaborative book is the Drupal developer documentation. In older versions of Drupal, only nodes of content type Book Page could be added to a book. However, since Drupal 7 nodes of any content type can be part of a book.
A Forum topic defines a topic for a forum discussion; people can reply to the topic by using comments. Forum nodes are organized into subject areas via a Taxonomy (list of categories).
A poll is a question with a set of possible responses. Once created, a poll automatically provides a simple running count of the number of votes received for each response.
Your own custom content type
You can create your own custom content types by going to Menu > Structure > Content types > Add content type (admin/structure/types/add). You might do this as a way to organize your content. For instance, you might have "Info" and "News flash" as two simple content types on your site, rather than just using "Article" for both. Additionally, creating your own content type means that you can decide what fields it has. When creating a new content type, you can customize what fields to add: an image, a poll, text, etc. These fields can appear as text boxes, select lists, check boxes, etc.
Content types by contributed modules
Finally, contributed modules define their own content types. Check the Download & Extend/Modules page of the Drupal handbook for more information on locating an appropriate module for your needs.
Content types in Drupal 5 and Drupal 6
As mentioned above, the default content types in Drupal 5 and 6 are Story and Page. Field management is included in Drupal 7 & 8's core content types. If you want to add custom fields to your content types in Drupal 5 or 6, install the Content Construction Kit (CCK) contributed module. Custom fields are used to store additional information beyond the Drupal defaults (title, body, authoring information, time created/updated, and publishing status); for instance, on a real estate site, a real estate listing content type might have fields for the type of property, land area, etc. Additional information on the CCK module is available from the Content Construction Kit Handbook.