Subnet Network: The physical Media, routers, and switches, that transfer data between smaller subnetworks in a larger network.

The subnet is a network that is a component of a larger network Subnet address: An extension of the IP addressing scheme allowing a site to use a single IP network address for multiple physical networks Subnet Mask: A bit mask used to select bits from an IP address for subnet addressing. The mask is 32 bits long and selects the network ID portion of the IP address and one or more bits of the local portion.

If that explanation was too complicated, try this one:
A network can be subnetted into one or more physical networks which form a subset of the main network. The Subnet Mask is the part of the IP address which is used to represent a subnetwork within a network. Using Subnet Masks allows you to use network address space which is normally unavailable and ensures that network traffic does not get sent to the whole network unless intended. Subnet Masks are a complex feature, so great care should be taken when using them.

SUBNET MASKS AND SUBNETTING
Subnets are logical subdivisions of a single Internet network. For technical or administrative reasons, it is desirable in many organizations to divide the network into several different networks. Routers then connect these independent networks. However, each organization that wishes to connect to the internet can usually obtain only a single Internet number. If multiple TCP/IP networks are interconnected across routers, you MUST assign a different network number to each network. However, if the network is part of the Internet, you cannot arbitrarily select any netwok number. Since network numbers must be assigned by the NIC. Subnet addressing allows an organization to use a single Internet network number for multiple physical networks. Subnets may be used with any class of Internet addressing except Class D (multicast).

A subnet mask allows the host portion of an Internet address to be divided into two parts. One part is used to identify the subnet number, and the other part is used to identify a host on that subnet. A host or router uses the leading bits of an IP address to determine its class. Once the class of an address is determined, the host can easily distinguish between the bits used to identify the network number part of the address, and the bits used to identify the host part of the address. How can a network element determine which bits from the local host portion of the address are used to define the subnet number? The ansewr is that a 32 bit subnet mask is configured to allow the host to make this distinction. The bits in the subnet mask and the Internet address have a one-to-one correspondence. The bits of the subnet mask are set to 1 if the device examining the address should treat the corresponding bit in the Internet address as part of the original network number or part of the subnet number. The bits in the mask are set to 0 if the device should treat the bit as part of the subnet host number. In other words, after the class of an IP address is determined, any bit from the original host number that has a corresponding bit set in the subnet mask is used to identify the subnet number. It is recommended that the subnet bits be contiguous and located as the most significant bits of the local host address.

LINKS:
Entering a subnet mask
Presentation of various subnets