CouchDB is a document oriented database. A document oriented database stores information as documents of related data. All of the data within a document is self contained, and does not rely on data in other documents within the database. This can be quite a shift if you’re used to working with a relational database, where data is broken up in to multiple rows existing in multiple tables, limiting (or eliminating) the duplication of data.
Database sharding is a method of horizontal partitioning in a database or search engine. Each individual partition is referred to as a shard or database shard.