In Reason CMS, everything is an entity. A page is an entity. A news story is an entity. An image is an entity. A video is an entity. A form is an entity. Basically, any type of content is an entity. (You can also think of an entity as an "entry", a "database row", or a "record".)
Reason creates powerful pages by connecting (or associating) entities together. For example, lets say you have a simple page on your site (an entity) and an image (also an entity) that you’ve uploaded to the site. To put the image on the page, simply associate (connect) it with the page.
Change the entity, and the change appears everywhere where the entity displays. Let’s say you placed the same image on 10 pages. But then you realize the image wasn't properly cropped and you need to swap it out. Do you have to go back, upload a new image, and re-associate the image with all 10 pages? Nope, nope, nope! All you need to do is edit the image entity that has the error, swapping out the old image for the new. The updated image will now show up in every place where the original image was. In other words, the entity remains the same, but the content of that entity is different.