A digital signature acts as a seal on a letter that proves it hasn't been opened since it was sent. The sender has a special seal (private key) to close the letter and anyone can check if it's still sealed properly (with the sender's public key). A digital certificate, on the other hand, is like an ID badge that proves who the sender is. It's given by a trusted group (Certificate Authority) and helps you trust that the letter (PDF file) came from who it says it did.