In traditional querying, the tables and cubes that are being consulted are stored on disc. "in-memory” solutions load all the data into RAM memory, from where it is being accessed by the application.
The advantage is the higher speed of querying and a reduced time of data modeling. Also structures are less predefined and easier to change. The approach obviously requires extra memory hardware requirements.
In memory solutions are in the process of replacing OLAP techniques, also due to the lower cost of RAM memory. Popular solutions include QlikView, TM1 and Powerpivot.