Entity-Relationship Model is the famous model of databases and their entities, properties and relationships.
At first, it looks a simple model if we concentrate in the reality. This is true, if the properties of each entity and the relationship between entities are realy consistent with truth at any condition. It is important to take care about the names used for each entity and the point in wich they can be broken into other entities to avoid duplicate data.
Although, there are details that I have saw recently: I think there is a natural trend for modelers to link some entities to entities that represent actors (people) instead of linking where they really would need to be linked (to preserve the referential integrity).