InfoQ: Graph Databases, NOSQL and Neo4j
NOSQL (Not Only SQL) really is a very wide category for a group of persistence solutions which don't follow the relational data model, and who don't use SQL as the query language.
In short, NOSQL databases can be categorized according to their data model into the following four categories:
1. Key-Value-stores
2. BigTable-implementations
3. Document-stores
4. Graph Databases