Key-Value DBs are like RDBs, but you can only have 2 columns: ID and VALUE.
Wide column DBs are like RDBs, but you may discover that some columns may suddenly go missing.
Document-oriented DBs are like RDBs, but you don't have JOINs (not entirely true for MongoDB though) and you can only store JSON in every row.
Graph DBs are like RDBs, but you get insane query language and relationships between nodes are also entries in the dedicated table.
You could even say that document-oriented DBs are like Key-Value DBs too. Just the "id" property of the document is the key and the entire document is the value.
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Key-Value DBs are like RDBs, but you can only have 2 columns: ID and VALUE.
Wide column DBs are like RDBs, but you may discover that some columns may suddenly go missing.
Document-oriented DBs are like RDBs, but you don't have JOINs (not entirely true for MongoDB though) and you can only store JSON in every row.
Graph DBs are like RDBs, but you get insane query language and relationships between nodes are also entries in the dedicated table.
Something like that.
You could even say that document-oriented DBs are like Key-Value DBs too. Just the "id" property of the document is the key and the entire document is the value.