difference between network id and host id??

what is the difference between net ID and host ID??
in a class C IP address say, 192.52.56.8 the first 3 octets denote the net ID while the last octet shows the host ID. what are the 2 used for ?

again in a class B IP address say, 172.52.36.5 the first 2 octets show the net ID while the last 2 denote the host ID…. what is the significance of the netID and hostID???

i got this definition for host ID frm google search, but i still didn’t understand …