PosDB is an engine of a disk-based column-store DBMS designed for processing OLAP queries in a shared nothing environment. It is written completely from scratch and aims to become a platform for studying the distributed query processing in column-stores. This paper presents the first comprehensive description of the system. The presentation begins with the history of column-stores in order to clarify the reasons of their success. Next, the creation of a new system is justified, and an overview of its architecture is given. Finally, all its components are described in detail. Currently, query execution in PosDB is based on the Volcano model with block-oriented processing and late materialization. Various physical operators have been developed for relational operations such as join, aggregation, and selection. Some auxiliary operators were developed to support intraquery parallelism and network communication. Data distribution is achieved using horizontal range partitioning and data replication. The current version of PosDB can execute all queries from the Star Schema Benchmark in both centralized and distributed environments.