Default Storage mode is MOLAP.In this, data gets pulled from Relational DB and pushed to Analysis services after aggregations are created.It fetches only leaf level attributes. Super fast but consumes more memory and its not real time. But using proactive caching MOLAP can be made realtime.

ROLAP is slow but real time and data and aggregations(stored in indexed views) are stored in relation databases.

HOLAP: Aggregations are stored in Analysis services and leaf level data are stored in relational data. It is realtime and slow for leaf level data and fast for aggregations