If you combine multiple classful networks it is supernetting, when you make them smaller you are subnetting .

CIDR is a condition where you no longer consider the classful boundries. You no longer need to think of the words subnetting or supernetting really because everything is a simple division between network and host.

Supernetting

The idea is to use networks that are larger than a traditional (i.e., classful) network - especially, larger than a class-C network.

In computer networking Math and binary operation are used in calculating subnet masks or in subnetting or in supernetting etc.

SUBNET: subnet is a group of networks having same network id. subnet increase the number of hosts with same network address, as result it reduces the wastage of addresses.

In subnetting bits of hostId will be captured by networdId.

SUPERNET:

supernet makes all the hosts with in a range of network address belong to a same group. This reduces the entries in the network address table.

Supernetting combines the octets of the address.