What exactly are Time Series and Order series?

Hi

I am a newcomer in SAP APO SNP. I have had only slight informal training in this. I wanted to know:

1. What exactly are time and order series? The only thing I know is that in Orders, data is stored with reference to orders, and in Time Series, data is stored without any reference to orders.

2. What are these "orders" which are taken as reference to store data in Order series?

Additional question: How is liveCache different from regular storage/database ?