OK, here are non-technical answers to your questions:-
It's all about a type of Linux based server that's easy to set up because pretty well everything you needed is right there inside, already configured and ready to run. These servers appealed to web hosting companies because they were so easy to set up and maintain. They were also an attractive proposition for small companies which needed a server but didn't have a dedicated IT team.
Sun Microsystems have discontinued their Cobalt servers - their new range is called 'Sunfire' and offers some advantages over the Cobalt servers. Cobalt continues to have a dedicated band of followers, partly because they are so easy to handle, and (I suspect) partly because they look so good with their blue cases and funky green LCD panels.
Why do you ask?