Why is security needed on the Internet?

What does security provide?

Identification / Authentication:
The persons / entities with whom we are communicating are really who they say they are.

Confidentiality:
The information within the message or transaction is kept confidential. It may only be read and understood by the intended sender and receiver.

Integrity:
The information within the message or transaction is not tampered with accidentally or deliberately while en route without all parties involved being aware of the tampering.

Non-Repudiation:
The sender cannot deny sending the message or transaction, and the receiver cannot deny receiving it.

Access Control:
Access to the protected information is only realized by the intended person or entity.

All the above security properties can be achieved and implemented through the use of Public Key Infrastructure (in particular Digital Certificates).