When I started my Crytal version game I went to Cherrygove with my Cyndaquil and entered the PokéCenter. Then I took the stairs to the second floor and talked to the man in a blue outfit on the left corner and he said to me:
"Hello ! You're ___, right ? I have something for you. (Choose Yes/No - I chose yes) Here you go ! "
And I recieved the "Teru-Sama". I check the items pocket and the item's descriptions is: ?
It also seems that I can get as much as I want of it. And I can actually sell it at the Mart for 19660 $
What is Teru-Sama ? What do you use it for ? I never hacked my game, went on a nintendo event or whatever ...

I assumed it was the GS Ball in Gold & Silver, but I did some research before I posted (you probably should have done that too), and came to the conclusion of: it's a test object. If you know how "MissingNo." shows up in RBY (every Pokemon/item/etc. is assigned a hexadecimal from 00 to FF), then you should understand why it's in the game, and what it's purpose is; absolutely nothing.

Since I don't trust the sources I found that suggest that the Teru-Sama does nothing, I did my own test. I found a gameshark list and loaded up my Silver Version save on my PC, entered the code, and it was like you described. It does nothing, and I'm pretty sure it's just a test object. It probably wasn't translated into English, because it was either just a test, or it was actually a item scrapped from the game.

It reminds me of '?' (yes, it's an actual item) that does the same exact thing - nothing. I used to have tons of ?'s because on my actual Silver Version cartridge I used a Gameshark.

As for how you got the Teru-Sama, it was probably just a random glitch. RBY/GSC are filled with them.

If some random idiot here tells you you can get Celebi etc. with it, don't believe them. Unless they have evidence to prove it, they're just trying to start rumours.

EDIT: I did some thinking, and now I'm starting to lean towards the theory that it was a scrapped item. The word 'Teru-Sama' has an actual meaning, so it's more likely an item than a test. I think ? was the test object. This is just my theory though, I'm telling you I'm 100% correct. Nobody on the internet will never know for sure, since all other evidence of what it is was either removed or never programmed into the game. By the way, if you're interested in scrapped ideas in G/S/C, look here.

WOW!!! Thanks for the answer !!!

EDIT: http://www.trsrockin.com/terusama.html
It says that Teru-Sama is the Japanese version of the GS ball. Is it true ??

Is it obtainable in any other game ??

In the Japanese version it was GS Ball. Some scraps of data from the GS Ball were left there, proving the Teru-Sama = GS Ball theory correct. They just didn't have enough of the GS Ball's data there for it to work.