In recent years, thanks not least to aggressive marketing by QlikTech (or Qlik Technologies as the are now often called) Tableau and Tibco Spotfire, columnar databases and in-memory databases have become very fashionable. Microsoft's VertiPaq engine, which is behind the PowerPivot product, is a good example of a tool that came in on the wave of this trend.
One of the results of these is that there seems to be some confusion about what the terms "in-memory" and "column oriented" mean, and attributes of one are often attributed to the other.
Just to be perfectly clear: A columnar database is not necessarily in-memory, and an in-memory database is not necessarily columnar.
In-memory is a somewhat vague term, since, as Nigel Pendse likes to point out, all databases have to hold data in memory to process it -- the CPU cannot directly access the hard drive. However, I would say that unlike some other tools, IBM Cognos TM1 and QlikView are in-memory. These products load everything into memory before they do anything. If there is not enough memory to fit the entire data set, the load fails and that's that. The same applies to SAP HANA. But unlike QlikView and HANA, TM1 is a multi-dimensional database.
The loading behavior of an in-memory database is much different to the MOLAP engine in Analysis Services, which is fundamentally disk-based but has sophisticated paging abilities to keep as much as the data as possible in memory, or the column oriented Spotfire, which attempts to load everything but uses paging if there is not enough memory.
Columnar is a much clearer and simpler term. It simply means that the data is stored by column instead of by row. There are a large number of analytic databases with this architecture, such as Exadata, SAND, Greenplum, Aster, or Sybase IQ, just to name a few. Some, like Vertica and VertiPaq, even refer to their columnar architecture in their names. Some columnar databases are in-memory, but many are designed to deal with huge amounts of data, up to the petabyte range, and cannot possibly hold it all in memory.
By the way, what got me off on this rant is actually this blog about Endeca Latitude 2 which actually equates the two technologies, and a Linked-In discussion the author started (which is private, so I can't link it here) with the title "Is Data Modeling Dead?"
The idea in memory databases kill data modelling comes from the fact that columnar databases are often used to discover hierarchies, and a whole generation of so-called "agile" in-memory database tools use this method. But in-memory multi-dimensional databases are still around and still very useful for analyzing data on well defined structures such as financial data.