What is a digital signature?

"A digital signature or digital signature scheme is a mathematical scheme for demonstrating the authenticity of a digital message or document. A valid digital signature gives a recipient reason to believe that the message was created by a known sender, and that it was not altered in transit. Digital signatures are commonly used for software distribution, financial transactions, and in other cases where it is important to detect forgery or tampering."

Digital signature is an operation where system takes your private key and, using it in special mathematical operations, applies source digest value to your data (email, document).The produced digest value is unique for this data, and after encrypting with private key is called digital signature.

When somebody wants to check your digital signature, he/she takes your public key and using mathematical operation restore source digest value and creates another digest value for real received data. After that system compares source digest value with created digest value - if they are equal, then digital signature is valid and content was originated from you and was not changed.