Design documents contain application logic. Any document in a database that has an _id starting with "_design/" can be used as design document. Usually there is one design document for each application.

{
    "_id": "_design/example",
    "view": {
        "foo": {
            "map": "function(doc){...};",
            "reduce": "function(keys, values, rereduce){...};"
        }
    }
}

The example above defines a view named foo, which can be requested from the following path, assuming the database is named db:

http://localhost:5984/db/_design/example/_view/foo