A note is a set of field. A card is a specific way those fields can be displayed to the user. If you want to learn the elements for example, instead of having

A note is a set of field. A card is a specific way those fields can be displayed to the user. If you want to learn the elements for example, instead of having multiple notes of the form "element name->symbol", "element name->atomic number" and so on, you can have a note that includes the fields "element name", "symbol", "atomic number" and so on. The cards then specify which possible combinations to show.