We use SAP r/3 for single plant planning & execution only 
it is not possible to do cross plant planning in SAP R/3. 
For cross plant planning we use APO but after planning in 
R/3 we  have to  send the planning results to r/3 for 
Execution because execution is not possible in APO.
Apo means Advance Planning & optimizer.
SAP APO stands for Advanced Planner and Optimizer, which is 
one area of SCM and application component of MySAP product. 
Its is designed to provide solutions for companies in 
Demand planning, network design, supply network planning, 
production planning and global available to promise, 
transportation and scheduling.

the major difference in between sap r/3 and APO is that in 
sap r/3 excecuting of production orders or purchase 
orders,material withdrawal and costing takesplace,whereas 
in apo only planning can be performed,
Sales orders are only created in SAP R/3 whereas in APO it 
is not created.
IN APO planning is done for the critical materials using 
MRP type X0.