A Semantic Layer is a business translation layer that sits between the database and the end user. 

This means that Users can define their Reporting requirements using familiar business terminology 
instead of Data Modeller terms.
For example, 'Customer' instead of 'Party'.

We have also designed :-
Compoments in a Semantic Layer

Data Mart for Markets and Brands

Data Mart with Conformed Dimensions

a Dimensional Model / Data Mart which is very useful for providng data for Reports and 
Business Intelligence.

An interesting perspective is shown in the Information Framework from the TM Forum, and
here is Wikipedia's entry for the Telecomm Consortium's Frameworx Shared Information/ Data Model.