The scan rates and resolution are very different. Adapters are available. You need to remember that the RCA (NTSC or PAL) resolution is worse than 1/3 of a modern computer.
They're also called scan converters. As mneary said, they're about 1/3 of the available resolution/refresh of a PC monitor so text is often bleary/twitchy, and video often flickers and is of poor quality. Modern PC video cards have a DVI output which can interface directly with an HDMI monitor/TV All that is needed is the appropriate cable and connectors.