A digital signature is quite different from a normal signature.
Someones normal signature is always the same, and independent
of the document signed. However, a digital signature is different
for different documents. It's closely related to encrypting
the document, except that in this case, you don't encrypt
the plain text, but you encrypt a digest, and attach the
encrypted digest to the document. The digest can be decrypted
using your public key (assuming you are using some well-known
public/private key encryption technique, like PGP).
Assuming the encryption algorithm can't be broken, and your
key isn't compromised, noone else can create the signature.
The signature can be copied, but the signature is worthless
without the plain text, copying to another document doesn't
sign it. Also, if the original text is modified, it no longer
matches the signature, so it prevents someone from modifying
the original document and pretending you signed it.