Subtypes are a decomposition of an entity into its various types. A relationship, as the name suggests, is a description about the relationship that exists between two entities. Information about how the entities relate, in particular, the optionality and cardinality of the relationship is modeled. A relationship should only be modeled when the relationship has relevance. If one desired, any entity could loosely be related to any other entity, but this is not the intention of modeling relationships.
Attributes are details about a specific entity.
These details provide greater clarification about the data that can or will be captured regarding an entity. One must be careful not to confuse entities and attributes. Entities can exist without attributes, but attributes can not exist without entities. There are many nomenclatures for the actual diagrams. For demonstration purposes, this article will adopt the Crow's Foot diagramming technique.