Evaluating bank_server:start() creates a global
process named bank_server. The local variable Data
contains a data base of {Name, Balance} tuples which
store the name and balance of the customer's account.
The bank server is accessed by sending it
{From, Request} messages. From is the Pid of the requesting
process, this is included so that the bank process
can send a reply to the requesting process (otherwise
there would be no way of knowing who had sent
the message).