Zend certified PHP/Magento developer

Explain meaning of column Schedule Status on the Index Management admin page

There are 2 columns on the Index Management page: Status and Schedule status.
Could you please provide detailed explanation what does each combination of values mean?

For example:

Status Schedule status Meaning
Ready Idle (0 in backlog) It means: Indexer is ready. 0 not-processed items in changelog.
Processing Suspended (0 in backlog) info about it

Index Management grid

I am confused by conbinations from the picture (Ready + Working (0 in backlog))