A good question one could ask is "With all the relational database management systems available today, why do we need another one ?". The main reasons are:
The key feature of NoSQL (and other similar packages mentioned in this manual), is its close integration with UNIX. Real-world problems are typically more complex than the data models provided by many DBMS. Actual applications, and Web-based ones are no exception, are complex puzzles made up of many small pieces, several of which are data-related. Unlike other fourth generation systems, NoSQL is an extension of the UNIX environment, making available the full power of UNIX during application development and usage.
NoSQL was designed with the UNIX shell language as its user interface. This level of integration removes the need to learn yet another set of commands to use and administer the database system. In NoSQL's view a database is just a file (or a set of files), which can be maintained like all other files that the user owns or to which he has access. Because NoSQL commands are executable programs, the UNIX shell is inherited as the primary command language of the database; no other proprietary database scripting language, to my knowledge, is as powerful and flexible as the UNIX shell. The shell-level nature of NoSQL encourages casual use of the system, and successful casual use leads to familiarity and successful formal use. This concept is much more thoroughly explained in the paper "The UNIX Shell As a Fourth Generation Language", that shows why the UNIX shell is an excellent tool for scripting database access.
NoSQL certainly cannot be seen as a "Big Name Database." But NoSQL's extreme generality and absence of restrictions make it more convenient and more effective for many everyday data management tasks than supposedly more powerful products.
Strictly speaking, NoSQL isn't a DBMS in any fashionable sense, if by DBMS one means a complex monolithic piece of software that sits on a local or a network socket listening for connections by client programs. By contrast, NoSQL is just a set of shell utilities meant to manipulate ordinary text files and relate them to one another in a database-like structure. As such, NoSQL does not provide any of the features that are usually found in most current RDBMS implementations, such as referential integrity, triggers, stored procedures or anything like that. I guess that NoSQL will look old-fashioned to many, a sort of vintage DBMS of little use, but it is meant to be like that, and that is precisely what I find more attractive about its underlying philosophy.
1 NoSQL has only been tested with mawk(1), which is Mike Brennan's implementation of the AWK programming language, and GNU AWK gawk(1). Making NoSQL work with other AWKs may prove hard, if at all possible.