Thanks you so much dear shorena for your exact and useful answer.
another thing to know.
I found my private key at blockchain.info.
you told that anybody who knows your private key can spend your bitcoin in whole? righ?
but how? in where?
Yes, anyone that knows your private key can spend the funds associated with it. Almost all wallets have an import feature, which would allow the wallet to create a transaction for you that spends the funds. In theory you could even do this by hand or write a little script that does that.
I wanted to test it. my private key at blockchain.info is available in some formats : bitcoin-qt ,hex,etc.
which one of them is in use?
"in use"? I dont understand that question.
for example the multibit ask me to import a " file" from my computer while
all of the mentioned formats of private keys are some letters and number that can be only copied and pasted in a text document
which multibit can not recognize . or can a private key be used to import in only the same service?
This depends how your wallet exports. E.g. multibit exports (when unencrypted) a file like this:
# KEEP YOUR PRIVATE KEYS SAFE !
# Anyone who can read this file can spend your bitcoin.
#
# Format:
# [[]]
#
# The Base58 encoded private keys are the same format as
# produced by the Satoshi client/ sipa dumpprivkey utility.
#
# Key createdAt is in UTC format as specified by ISO 8601
# e.g: 2011-12-31T16:42:00Z . The century, 'T' and 'Z' are mandatory
#
L35DEFBrrMJHVUquyyvJn9ZiXfwbwKBJBeEbRkXwnqhbjAjnrj23 2014-12-04T23:16:20Z
# End of private keys
You can just create the same format with your unencrypted private key from bc.i and multibit can import it without problem. AFAIK bc.i exports encrypted, thus multibit might not understand the data.