The server function contains the instructions for the reactivity needed for the Shiny app.
The following function takes two arguments:
objects are created by Shiny when the session begins and passed to the server
input is used to pass the control values, in this case
input$obs the number
of observations. Notice how it is used to generate random numbers from
a Normal distribution:
output object contains the output objects, in our case the rendered plot.
output together describe the state of the app.