Exceptions prove the rule

Like other programming languages, Apex includes a built-in class for exceptions. When your code exceeds a limit, an exception is thrown. For this reason, you really need to understand how exceptions work in Apex before we can go any further on limits. When the Salesforce1 Platform throws an exception, all statements that have been executed within that transaction are rolled back causing your entire attempt to execute code to fail. It's not fun to get an exception; it means that an end user is upset and you have more work to do.

Apex includes a generic exception class as well as over 20 types of specific exceptions for when various rules are broken. A lot of these specific exception types are easy to avoid such as don't divide by 0 when doing math, don't refer an index of a list that is out of bounds, and don't construct an account and then add it to a list of contacts. You get the picture. When your code does cause one of these exceptions, you can usually avoid it...