Blocks are the navigational or content additions that generally live on the left or right side of a page when you view it in your browser. Block placement can be placed elsewhere via the admin settings. Blocks are not nodes, they are just a way of positioning data within a page. The look of blocks can be controlled by each theme by defining the block($subject, $content, $region = "main") method.

A module is software (code) that extends Drupal features and/or functionality. Core modules are those included with the main download of Drupal. Contributed (or "contrib") modules are available for separate download from the modules section of downloads.

Almost all content in Drupal is stored as a node. When people refer to "a node" all they mean is a piece of content stored within Drupal. A node could be a poll, a story, an image, a book page, etc.

Permissions control access to content creation, modification and site administration. Administrators assign permissions to roles, then assign roles to users. The first user id of a Drupal site (uid=1) automatically receives all permissions, no matter what role that user belongs to.

Roles are sets of permissions that can be applied to individual users. Users can take on more than one role. Two roles, authenticated user (those users that sign up for an account) and anonymous users (those either without an account or not logged in) are supplied by default with Drupal installations, but they can be configured and the first user can create additional roles.

Taxonomy is literally "the science of classification". Drupal uses taxonomy to describe the category system, which you can use to classify and organize content on your web site. In Drupal a taxonomy is a set of categories. There is additional information on the taxonomy system in the documentation.

A theme is a file or (usually) collection of files (php, theme, css, jpg, gif, png), which together determine the look and feel of your site. These files are often used by one of the theme engines available for Drupal which is a PHP file of functions which turn arguments into HTML markup. Drupal modules define themeable functions which can be overridden by the theme file. There are additional themes available in the themes section of downloads.