Basically, there are 4 types of NoSQL databases:
1. Key Value Pair Databases
In this type, the data is stored as key value pairs. By that we mean something like
In the above arrangement the keys are Name and DOB and the values are what is given on the right. Examples of key value pair database include Riak, BerkeleyDB, DynamoDB etc.
2. Column Store Databases
This type of DB stores all information in massively distributed column repositories. Examples include Hadoop / HBase, Cassandra, Amazon SimpleDB etc.
3. Document stores
This database uses documents as its basic building block. Typically the data is stores as JSON documents. Examples include MongoDB, Couchbase etc.
4. Graph Databases
Here the entire database schema is traversed as a graph, giving massive tree like information about any object. Examples include Neo4J, InfoGrid etc.
Besides these there are other databases like:
Multimodel Databases, Object Databases, XML Databases, MultiDimesnional Databases and Multivalue Databases
Although each of these offerings have their own pros and cons, graph databases like what Facebook uses are considered more powerful than the norm. However usage will vary according to needs.