A dedicated server is one leased from a hosting company. They are responsible for the hardware, the operating system, web server software, and , within limitations, the software they load on the server.
A colocated server is different because the server itself is owned by
you the customer, who is responsible for all parts of it, from the hardware to the operating system.
Physical space and bandwidth are leased from the hosting company, and their responsibility is solely the connection to the Internet and related matters. So basically you carry or ship your system to a datacenter.
