Similar to the dedicated web hosting service, but the user owns the colo server; the hosting company provides physical space that the server takes up and takes care of the server. This is the most powerful and expensive type of web hosting service. In most cases, the colocation provider may provide little to no support directly for their client's machine, providing only the electrical, Internet access, and storage facilities for the server. In most cases for colo, the client would have his own administrator visit the data center on site to do any hardware upgrades or changes. Formerly, many colocation providers would accept any system configuration for hosting, even ones housed in desktop-style minitower cases, but most hosts now require rack mount enclosures and standard system configurations.
But, again, operationally it's quite strong. 24/7/365 customer support is available not only by live chat and email, but by phone. It offers free site migrations with some dedicated attention paid to making a transfer as smooth as possible. And, if you're willing to go for one of the higher-end plans, the company has put some serious attention into performance and caching.