The Microsoft BI Semantic Model or BISM is a new term introduced with the release of SQL Server 2012. It refers to the way Analysis Services and PowerPivot can be used as a semantic layer, on top of other data, for reporting purposes. Rather than being a product on its own, it is a conceptual term to describe the different options you have within Analysis Services 2012.
With SQL Server 2012, the ‘Tabular Model' was introduced next to the already existing Multidimensional Model (based on cubes). The Tabular Model however is not build around a cube like the Multidimensional Model, but is based on tables, relationships, in-memory storage (VertiPaq) and columnstore indexes.
So actually, the term BISM is more or less the same as the term Analysis Services, which since 2012 has 2 types of models to work with. The Tabular Model that is faster and easier to develop and the Multidimensional Model, which has more advanced options and features.
An insight that discusses the differences between the Tabular and the Multi-dimensional Model can be found here.