PCs: Keyboards,mouse,screen,motherboard,CD/DVD ROM
PLCs: Don't have these stuff
PCs & PLCs: I/O units,memory(different size though),CPU
That's all I know...
As far as the other question is concerned...
"efiiciency of switching contacts"...plc have many internal devices such as timres,counters etc giving us the opportunity not to use external stuff and make less wiring..
"aving installation time"...I understand this "term" like this: when we need a change in automation,all we have to do is re-programm tha PLC instead of re-wire the hole ...