Often your site will have multiple content types that you can choose from when creating content. Often users will simply create "Articles" or "Basic Pages" for new content, however "Photo Galleries" and "Slideshow Images" will often have their own content types. More advanced sites will have multiple extra content types. These types are created to create custom functionality.
Customer Testimonials: Testimonials will often be automatically listed on a page, or a block in a sidebar.
Photo Gallery: A common content type, creates a new gallery with fields to upload the images for that particular gallery. The Gallery will then be added to a list of galleries with the first image in the gallery being used as a thumbnail and linking to the full gallery where you can view all the images.
Slideshow Image: Another common type. This content type often will have an image upload field, and the image will be automatically added to a slideshow on the site.