I learnt big-O-notation in analytic NT books but in papers they usually use $\ll$ instead. My friend told me they are absolutely same.
1- I wanted to be sure if they are same? and
2- if so just curious why there are two different notations?
3- How to pronounce $\ll$ in talking? and $\gg$? (which has no similarity to O notation I think).
4- How to pronounce $f \asymp g$?
PS There is a similar question here but unf it doesn't answer all questions.
1) Your friend is right: The two statements $f = O(g)$ and $f \ll g$ mean exactly the same thing, namely that there exists some $C > 0$ such that $|f(x)|\leqslant C\cdot g(x)$ for all $x$ large enough, small enough, or...
3) I'd say this out loud as "$f$ is at most a constant times $g$."
2) In my experience the $O(\cdot)$-notation is more convenient in certain manipulations than Vinogradov's notation: One can write \begin{align*} \pi (x) = \frac{x}{\log x} \left( 1 + O\left( \frac{1}{\log x} \right) \right) \end{align*} using the $O(\cdot)$-notation, but this cannot really be written as elegantly using Vinogradov's notation. On the other hand, Vinogradov's notation has a very nice and clean look and so it's sometimes preferred for typographical reasons. So each notation has its advantages and disadvantages.
4) I'd say, "$f$ and $g$ have the same order of magnitude."