What is a digital signature?

What is a digital signature?
Digital Signature is a mechanism for providing an extremely high level of trust that a signed data file is authentic, and tamper-free, typically using PKA. It is believed to be highly resistant to forgery, unlike written, physical signatures.

Users need to have a Digital Certificate (with a public key) issued by the Valid Certifying Authority like Verisign,Thwate etc. Once the user gets the Digital Certificate (and corresponding secret key) then he will have to get the Digital Signature software which will use the details from the Digital Certificate to insert digital signature on the electronic documents(WORD, EXCEL, PDF etc).
Digital signature provides Authentication, Data Integrity, and non-Repudiation to the documents, contents and transactions. It may also be used to support confidentiality (encryption, scrambling), primarily by exchange of symmetric keys used to encrypt and decrypt the data of interest.