How do you define "power" in One Piece

[docdan63]

Discussion in this thread will be about content in One Piece anime beyond what Kaizoku-Fansubs has released

I was looking at the description of the 3 great powers in OP. And it got me thinking. Since the Marines obviously have seastone cages and huge ships and giant groups of men ready to do battle. The shichibukai have previous bounty men of 80-200 million a piece and their respective massive amounts of power. And the the last one got me thinking the most. They're called the Yonkou literally Four Emperors, are the 4 great pirates who rule the second half of the Grand Line (known as 'the New World'). This was a mystery to me. They consist of the most powerful pirates in the series. So far 3 have been revealed.

http://onepiece.wikia.com/wiki/Yonkou

Shanks
White beard
Kaidou (he beat Moria in the New World. This has yet to be revealed as how)
There is one left to be identified. And we don't know which one is Kaidou yet.

Now what I'm wondering is this. How can the Yonkou be so strong? How is shanks so strong? How is white beard so strong (besides being a giant) Do you think they have powers? Can they have some special crew to back them that no ones crew but Rogers could come close too? Do they have Fruits?

I'd like to know your comments