Your digital certificate allows you to participate in secure on-line transactions and communications.  Your digital certificate is your on-line identification.  While others can view your certificate information, using your digital certificate should be kept confidential. 

Your private key (which is usually password protected in your web browser) is literally the key that opens your digital certificate.  It allows you to digitally sign documents and decrypt information that was only meant for you.  You should safeguard your private key just as you would any other form of identification. 

Just as you would not allow someone else to sign your name to something, or to use your social security number, you would not allow others to use your digital certificate.