Informally it can be defined as how you explain what has happen to you; do you think events are personal, permanent and pervasive?

• Personal. People experiencing events may see themselves as the cause; that is, they have internalized the cause for the event. Example: “I always forget to make that turn” (internal) as opposed to “That turn can sure sneak up on you” (external).
• Permanent. People may see the situation as unchangeable, e.g., “I always lose my keys” or “I never forget a face”.
• Pervasive. People may see the situation as affecting all aspects of life, e.g., “I can’t do anything right” or “Everything I touch seems to turn to gold”.

People who generally tend to blame themselves for negative events, believe that such events will continue indefinitely, and let such events affect many aspects of their lives display what is called a pessimistic explanatory style. The definition of pessimistic used here is different from the connotations that usually go along with the word.
Conversely, people who generally tend to blame others for negative events, believe that such events will end soon, and do not let such events affect too many aspects of their lives display what is called an optimistic explanatory style.  Again, the definition used here is different from the usual connotations of optimistic.
It is important to note that the concept of explanatory style encompasses a wide range of possible responses to both positive and negative occurrences, rather than a black-white difference between optimism and pessimism. Also, an individual does not necessarily have the same explanatory style in all aspects of life, but may exhibit varying responses to different types of events.