what affects core choice?

[square_imp]

having just sorted my new machine out over the last couple of days I have been wondering about multi-core cpu structure. What makes a certain program use a certain core of your processor while another uses a different core? I have noticed during some programs that one of the cores in my quad is used a lot more than another and not always the first core. Do the different cores do different tasks or what?

Just my random ramblings......