How to tranfer data between two application

Hi,

 I Have  two applications and  these have individual databases

1) Employees
2)Customers
 
here customers application viewpage.aspx needs employeeid from Employees Application
So how i send employeeid  from Employees application Customers app.
 
and  i want know how we accomplish  if two or three applicatons want to communicate for data or some information

 
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