What are the four different types of errors?

 There are several different factors that combine to affect the overall solution accuracy.  
In order of magnitude:

 Round-off errors
 • Computer is working to a certain numerical precision.

 Iteration errors
 • Difference between ‘converged’ solution and solution at iteration ‘n’.

 Solution errors
 • Difference between converged solution on current grid and ‘exact’ solution of model equations.
 • ‘Exact’ solution → Solution on infinitely fine grid.

 Model errors
 • Difference between ‘exact’ solution of model equations and reality (data or analytic solution.