The world has started to rely increasingly on software that is becoming more and more complex. Today software is everywhere - part of every system or machine that we use in daily life. Software Quality and Reliability have become an important concern of the software industry. While improved and formal software development processes help avoid more and more defects, a large number of defects are still left in software. In one of history's well-known bugs, the entire long distance of AT&T was down for nine hours. The melt down was finally traced to a single line of code that could have been best detected by some effective unit testing. Beizer argues that the number of bugs left behind after a software engineer "delivers" his code to others is still too high for comfort. While improved design techniques try to avoid defects, effective testing continues to play an important role in removing defects left behind and is the cornerstone of the Software Quality Assurance activity.................
More at http://www.deccanetworld.com/sublinks/whitepapers/unit_testing.htm
It is very good white paper written by “R.Venkat Rajendran, Director, Deccanet Designs Ltd.”