I've explained that the netmask indicates the bits that are the network part of the address, and that changing anything in those bits puts you on a different network or subnet.
Any network is a subnet of something larger, so in that sense, the terms are identical.
Or, your network is the bits you cannot change because someone else assigned them to you. As it's always just the number of bits that is important, we can represent networks or subnets another way. Those are pretty easy. If you aren't used to thinking in bits, this might give you a little headache. But don't panic, it's not that hard.