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?
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?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?