Independent Software testing or IV&V as most people call it, is becoming more and more relevant in today's world. Few years back, it would be the developers who would carry out the testing which was very ineffective.
Mindset of testing has to be very customer oriented. A tester needs to think and act like a end user. Also, a tester needs to be up to date on the common mistakes that developers are likely to make.
An expert tester is well versed with Software testing concepts and methodlogies which ensures all defects are found and fixed...