There are three main types of errors:

Syntax errors: these occur when something is misspelled or something is missing that should be there.  Usually visual basic will let you know about syntax errors as you write your code.

Run-time errors:  occur when you try to run your project.  The project will stop and tell you it has found an error.  

Clicking the debug button will highlight the line of code where the program stopped.  You now need to study the code carefully to see if you have misspelled something or made another mistake.

Logic Errors:  are the hardest to track down.  These errors won't stop your program from running, but may cause it to give unexpected results.  A logic error might be to make an image invisible when you expected it to be visible.  You need to carefully trace through your code to work out where you went wrong.