In the past, OLAP solutions were divided into MOLAP (multidimensional OLAP), ROLAP (relational OLAP), and DOLAP (desktop OLAP). New products have replaced the MOLAP and ROLAP solutions with a hybrid OLAP (HOLAP) approach.
Hybrid OLAP. A storage mode that uses a combination of multidimensional data structures and relational database tables to store multidimensional data. Analysis Services stores aggregations for a HOLAP partition in a multidimensional structure and stores facts in a relational database.
HOLAP (Hybrid Online Analytical Process) is a combination of ROLAP and MOLAP which are other possible implementation of OLAP. HOLAP allows to store part of the data in the MOLAP store and another part of the data in ROLAP store. The degree of control that cube designer has over this partitioning varies from product to product.