Public vs Shared variable

A 'Shared' variable is a variable which applies to a Class or Structure as a whole and not to any particular instance. In a sense, it is 'shared' between all the instances of its containing Class or Structure - hence the name. 

In a Module, all module-variables are Shared by default because it is not possible to create instances of a  Module.

'Public' is just an access modifier which simply means that the variable, function, class etc. to which it's applied is accessible from all other code in the same program or (in the case of a library) from code within a program which uses that library.

Notice that a variable can be declared both Shared and Public. If you don't use Public but just use Dim on its own, then it's Private by default. 

Notice also that a Public variable cannot be more accessible than its containing Class, Structure or Module. So, if the latter is declared as Friend (the default), then the Public variable will only be accessible within the same program.