The meanings may differ depending on the company. Every company can essentially define what a specific title does and it's not the same from company to company.
Wit that disclaimer here i go:
Help Desk i see as more IT related. Help Desk is tier1 support where someone calls into a help desk line because something computer related broke - software / hardware / phone / printer, ect.
Call center has nothing to do with IT, it's generally a department with a lot of people working the phone's doing anything from selling insurance, taking market research surveys to trying to get you to switch cable companies.
Call center is more of a general title that encompasses dozens of call center types. Help desk is generally IT related (all be in broad in it's own right).