As defined by Drupal, "a content type is a pre-defined collection of data types (fields) which relate to each other by an informational context". (See the Drupal website for more information about content types). To put it simply, a content type is a way of organizing information through the use of unique fields and displays to showcase a specific kind of content.
One website can house lots of content, in many different formats and pages. For example, many websites have news items, blog posts, job listings, etc., as well as standard, informational pages—and each of these can be created using different content types. Each one will have its own set of fields and a place on the website, and will have one or more displays, such as a list (this may look like a filterable directory), map, calendar, or detail view. Each content type can also look very different depending on how it was developed and designed, and the unique content needs of a particular website.