What a tester does?

My main focus is on what does a tester do?
Actually, on Sunday I met my social friends which are mostly developers as well as been in developing for a long time (experience, mini=1 max=5). They thought to blast me with their developing idea as we are aware, all the developers think testers are devils for them, God knows why and what makes them think that…
I tried to explain them the whole concept of testing and even why testing field has got a different position.
The place we met was CCD (café coffee day) and before this discussion started we had planned to leave that place within next half an hour but, would you believe that discussion continued for two hours with 25 coffees and 60 cigarette between 6 people (5 developers 1 tester) no one realized when, where and how time went on…

We all where enjoying the discussion because we included waiter, shop manager, parents, teachers, boy walking on road and even Gandhi-ji in examples to prove our respective points…
Slowly and steadily the topic started heating up and we all started shouting like any thing, believe me we all where like politicians shouting in parliament but, suddenly from some where the speaker (shop manager) came into picture and requested us to calm down, manager was a nice person coz twice he tolerated our noise but third time he pleaded us to leave the café so, we finished our topic and decided to meet up again on next Sunday with the same topic and same café. Left for our respective home (God save the ccd manager)

While returning back home I realized a pattern in different question asked by developers,
Eventually question where many but motive was the agenda, I went into flashback and re-winded all question and you know the entire questions where revolving around different patterns of questions which are as follows….
Agenda
1. Who is a tester?
2. What does Software testing mean?
3. Why testing came in software?
4. How testing should be done?
5. Why not developers can do testing?
6. Why do we need testers?
7. Value of testing?
8. Is testing good or bad?
9. Whom exactly testing benefits?
10. Why there is no qualification for a tester.
11. What a software tester does?

I explained them the entire 10 question but got a bit confused when question 11 came up to me, what a tester does? (And added the following point)
What actually a software tester do to a product, add value, add confusion, add in an extra brain or create misunderstanding. What a tester does? (I thought the best suitable answer would be “ Testing ” that is what a tester does isn’t it? )

But would like to here your comments on this question…
What a tester does in testing?