An entity acts as a wrapper for a particular set of attributes. The entity itself does not store data—the information is contained in the attributes. The entity simply defines an object as a particular set of attributes.
(Okay, technically, an entity is a sort of super attribute that contains a machine name, a human name, and some validation and permission rules. Again, this is important for developing a CMS, but not so important here.)
URI stands for Uniform Resource Identifier. A URI is a way of picking out a specific entity. The URI is how you pick out a specific thing, and how you distinguish that thing from different (if similar) things.
For a content strategist or a developer, content type is a technical term that refers to a specific set of content management system functionality. Roughly speaking, a content type (in this sense) is a set of attributes (or fields) that are collected into a single entity that is then published on the website.
Remember that business about entities being super attributes? This is where it comes back into play. If I have an entity called news and another called article, both of which contain the same set of attributes, then they aren’t really two entities—they’re subtypes of a single entity type.