What is the difference between open source and closed source in computing?

[Kross]

One of the concepts that I hear often in computing is always open source. I don't hear closed source much often. I rather hear proprietary software. What is the real difference between the two and what are the pros and cons of each of them?

 

[King Belieal]

The major difference is that the source code for open source apps are public and the source code for closed source apps are a top secret. Programmers can modify open source apps and possibly improve it while they can't do that with closed source apps.