Difference between OW, OB and DB layers in terms of Availability of data in terms of Time and Space

Dear All Kindly explain the sequence in which the data is read through CRM_ORDER_READ and kindly explain the sequence in which Data is written to Database using CRM_ORDER_MAINTAIN. Also humbly Request you to explain, difference of availability( in terms of time and user session/server logon session) of data in OW, OB and DB layers respectively. I am Eagerly looking for the answer in order to understand the basic architecture of Data flow in CRM. Waiting eagerly for your answers. Thanks