POD stands for Plain Old Data - that is, a class (whether defined with the keyword
struct or the keyword
class) without constructors, destructors and virtual members functions. Wikipedia's article on POD goes into a bit more detail and defines it as:
A Plain Old Data Structure in C++ is an aggregate class that contains only PODS as members, has no user-defined destructor, no user-defined copy assignment operator, and no nonstatic members of pointer-to-member type.