NoSQL Question:

An aggregate is a collection of data that we interact with as a unit. Aggregates form the boundaries for ACID operations with the database. Key-value, document, and column-family databases can all be seen as forms of aggregate-oriented database. Aggregates make it easier for the database to manage data storage over clusters. Aggregate-oriented databases work best when most data interaction is done with the same aggregate; aggregate-ignorant databases are better when interactions use data organized in many different formations.
Aggregate-oriented databases make inter-aggregate relationships more difficult to handle than intra-aggregate relationships. They often compute materialized views to provide data organized differently from their primary aggregates. This is often done with map-reduce computations.