Gallery…is specifically an exposition of images attached to a post. In that same vein, an upload is “attached to a post” when you upload it while editing a post.
Example: “Let’s upload images to the gallery as attachments, not as media files.”
When “gallery” comes to mind, an art museum might suffice. But gallery is just another word for “photo album” in WordPress. A gallery is a cluster of images that are assigned to a shortcode and displayed in a post as an organized bunch of images.
How do you place a gallery into a post? It is very simple. You can go to “Add Media” > “Create Gallery” and then upload new images or previous images to the gallery.
You can swap images around to a preferred order and add metadata to the images, such as a title and caption. There are also gallery settings where you can specify how many columns you want to display the images in and how large each image should be.
In the text editor, the gallery is not seen as a gallery view. It is seen as a shortcode, or a “macro code for use in post content.” The gallery shortcode is presented as gallery, featuring a string of ids that identify images placed in the gallery.
For an easy step-by-step article on how to place a gallery into a WordPress post, you can read the Codex’s entry on the WordPress Gallery.
For more information about the WordPress Gallery with regards to WordPress, there are some helpful links below: