TO: Pierogoal:

OB is organization block, especial OB1 is main program cycle block. OB1 is must be used in PLC.
FB (Function block) and FC (Function), you can consider them as subprogram. As example,you program a C=2a+b function in a subprogram FB1. You can use FB1 in OB1 several times with different data. The diference between FB and FC is FB should have a instance data block (DB), and FC have not. With DB, you can understand it as a speical memory area.

For details, I recommend you to read help files or Siemens manuals.