A local variable is only visible to that script or block of code on the same client depending on where it is defined and is presented with a "_" such as _myLocalVar = 0;
A global variable is visible to all the scripts etc on the same client, presented without the "_" such as myGlobalVar = 0;
A public variable is visible to all clients, variables are usually made public with publicVariable.
There is probably a better way to explain it but I have been drinking.