When you log in, your userID is saved, and only information relevant to that user is pulled (normally from a database) and displayed. The basic form is the same for all users, it is just the detail that is specific.
For example. If you create an application which has a form and in the load event you let them log in via a dialog:
frmLogin log = new frmLogin();
if (log.ShowDialog == DialogResult.OK)
{
if (log.UserName == "The Boss")
{
labGreeting = "Hello Boss! Shall we go down the pub, or actually do some work?";
}
else
{
labGreeting = string.Format("Hello {0}! Welcome back!",log.UserName);
}
}
else
{
Close();
}
Then if you log in with the name "Fred", or "Joe" you would see a personalised message, but "The Boss" would get a completely different screen.
You need to get yourself a book, and study it - there is way too much to explain in a simple answer!