There are several methods to communicate between a PLC and a programmer or even between two or more PLCs.
Communications between a PLC and a programmer (PC or Hand held) are provided by the manufacturers and you only have to plug in a cable from your PC to the programming port on the PLC.
This communication can be RS232; RS485, TTY, USB, CAT 5 etc.
Communications between PLCs can be carried out by dedicated links supplied/programmed by the makers (RS232 etc) or via outputs from one PLC to the inputs on another PLC.
With S7 this can be MPI to MPI and/or Profibus.
This direct link method of communication can be as simple as, if an output on the first PLC is on then the corresponding input on the second PLC will be on and then this input is used within the program on the second PLC.
If a word of input/outputs (16 bits) are used then numerical data can be transferred from one PLC to the other (refer back to the section on numbering systems).
There are many other methods of communication between PLCs and also from PLC to PC. Please refer to the manuals supplied with the PLC that you are using for full details on communications.