A suite is a collection of things.
A three-piece suite is a collection of a sofa and two armchairs.
A set of connected rooms can be called a suite.
The C-suite is a collection of people or job titles beginning with C or chief: CEO (chief engineering officer), CIO (chief information officer), CFO (chief financial officer) and so on.
An alternative to CEO is MD or managing director, but it seems people prefer to be chiefs rather than managers or directors and this list of titles is becoming quite long as more titles are changed to C- titles.
A 2012 Forbes article called "C Is For Silly: The New C-Suite Titles" said:
In the past few years, the c-suite has exploded its members, knighting nearly every department head with new, inventive chief titles likely dreamed up by the marketing team.
Kodak and Dell appointed Chief Listeners. Facebook recently added two Chief Privacy Officers. Coca-Cola is really gung-ho on the trend, employing a Chief Administrative Officer, Chief Sustainability Officer, Chief Scientific and Regulatory Officer, and Chief Quality and Product Integrity Officer, among others. Microsoft has a Chief People Officer; IBM a Chief Information Officer; Xerox a Chief Strategy Officer; and New York City has its very own Chief Digital Officer.
“It is all corporate Kindergarten playtime title-making,” says Mark Stevens, a marketing and management expert and author of Your Marketing Sucks. “It’s a puppet show. These people have absolutely no power.”
The only C’s with “real” power, Stevens says, are the Chief Executive Officer, Chief Financial Officer and, occasionally, Chief Operating Officer. “Most of these vanity titles don’t even report to the CEO,” he says.