Stealth address in technical level Lets say user A wants to send bitcoin to user B stealth address.


What is exactly user B's stealth address? Is it his public key? Since User A needs to multiply his private key with user B's public key based on Elliptic Curve Diffie-Hellman so I assume user A is receiving the complete point which is user B's public key.
User A shoud calculate PrivA * PublicB which results in a coordinate. As far as I know this point will be used by multiplying it to the publicB and a nonce and that will be sent to the network. If what I am saying is correct, how can I multiply this coordinate with PublicB since both of them are points. Do we only get one axis of this point from the diffie hellman's calculation?
What is exactly user B's stealth address? Is it his public key?


In the simplest stealth address scheme, yes. The exact encoding depends on the implementation; DarkWallet's is described in their wiki.


  how can I multiply this coordinate with PublicB since both of them are points?


Correct, S = PrivA * PublicB = PrivB * PublicA is a point. We want an integral shared secret. ECDH tells you to use the x coordinate of S as the shared secret, but in Bitcoin it's done differently. Instead int(sha256(compress(S))) is used:


serialize S in compressed form as per SEC 1's Elliptic-Curve-Point-to-Octet-String Conversion
take its SHA-256 digest
interpret the result as a 256-bit big-endian integer


This integer, c, is then used by A to calculate a public key PublicB + c*G to which A spends, and by B to calculate its private key (PrivB + c) * G from which B can redeem (where G is the base point generator).

Note that there's a problem with this "simplest" scheme. The only way that B's wallet software can see if a UTXO is redeemable by B (to keep track of the wallet's balance) is by doing the math above, which requires PrivB. In other words, it makes watching-only wallets impossible (and cold wallets impractical).

One solution to this is to use one PublicB/PrivB pair for ECDH, but a different PublicB2/PrivB2 pair for deriving the spend/redeem keys. Now you can keep PrivB in your watching-only wallet, and PrivB2 in your cold wallet. On the down side, B's stealth address is now twice as long, containing both PublicB and PublicB2.