New computers include a media player like QuickTime or Windows Media Player. Unfortunately, these players can't handle every type of video file.
VLC is the best alternative around. It's a powerful yet lightweight media player that can handle almost any media format in existence. The latest version also adds support for multi-core CPUs, GPU rendering and experimental support for Blu-ray discs.
VLC does almost everything you need out of the box. However, you can customize it heavily and add more advanced options. For example, VLC can also be used to stream media over a network.
Users of the Mac version get a new single-window interface and the ability to send content to iOS devices.