What Exactly Is An Active Partition

I’ve got Windows Vista loaded on both my SSD (drive C:, marked “Active”), yet I’ve got Vista loaded on my internal HDD too (drive D:) and it isn’t marked ‘active’, so the question is, why isn’t drive D: marked ‘active’?, plus. . . I’ve got a 1TB external USB HDD (Western Digital - My Book) that has two partitions on it, the first one is drive G:\Images and has Macrium Reflect images stored on it and it is marked ‘active’. The other partition is drive H:\Storage and it just for miscellaneous file storage and it isn’t marked ‘active’. Why is drive H:\Images marked ‘active’ when Windows isn’t loaded on it?

This is confusing me. Can anyone explain this to me? I would appreciate it.