What is the difference between a main panel and a sub-panel?
A main panel (or service entrance panel) is simply a panelboard that contains the main service disconnect for a property -- this can be up to six circuit breakers in a rule-of-six (split bus) panel, but is more commonly a single main circuit breaker or fused disconnect (such as a pullout).

Or in other words: the service from the electrical utility enters the main electrical panel for a property, and all other panels on that property are subpanels, as long as there is not a second utility service or a transformer on the property.  There are also other special cases such as multiple section main panels, but they're beyond the scope of this answer.