Unix systems implement privileged ports: ports lower than 1024 can only be used by the system administrator (root user). Considering that typical internet servers run on a low TCP port (for example, 25/tcp for an SMTP server or 80/tcp for an HTTP server), this limitation ensures that only the system administrator can run such servers.
Windows systems do not implement privileged ports. As a consequence, anybody can bind a TCP or UDP server on a low port. As explained later, this has some serious security implications.