As the name implies, columnar databases are organized by column rather than row: that is, all instances of a single data element (say, Customer Name) are stored together so they can be accessed as a unit. This makes them particularly efficient at analytical queries, such as list selections, which often read a few data elements but need to see all instances of these elements. In contrast, a conventional relational database stores data by rows, so all information for a particular record (row) is immediately accessible. This makes sense for transactional queries, which typically concern one record at a time.

Source: How to Judge a Columnar Database -Marketing Systems
Information Management Magazine, December 2007  by David M. Raab