Tagging is a simple way to relate data. Also referred to as "labeling" or "folksonomy", it's simply a way to connect a piece of data with a word or phrase in order to make it easy to connect related data.
If A+tag points to B, then
B+*tagged will return A
By default +tag cards automatically have the type Pointer, which provides an interface for adding words and phrases, which Wagn will then treat as card names.
There is also a built-in search for +*tagged cards which will return the tagged cards.