Databases that propose alternative solution to SQL (Relational) Database which can be document oriented, key/value, graph, objects,...
NoSQL Database means "Not Only SQL" not "No SQL". Those are alternatives to classic relational Database which has been mainly designed to answered to the scaling/performance problem of SQL Databases by loosing some cons of SQL (consistency,...)
Unlike SQL which is standardized and fit only for relational databases. NoSQL database proprose various type of databases :
Be really carefull about comparing SQL to NoSQL, because since they're many type of NoSQL database and Many Distributor for each of this concept you can't just generally compare them bluntly.
If you need some information about which system to use, write what your system needs and answerers will guide you accordingly to your need.