Good question. To answer it fully would involve a pretty deep look at Ethernet Wiring. But I'll try to explain it in simpler language.
All three speeds (10, 100, 1000) run over the same physical wiring: Unshielded Twisted Pair (UTP). UTP is made up of 4 pairs of wires (8 total wires) -- each pair is twisted around each other. Each pair of wires work together to send signals across to the other end.
What enables the same physical cabling to carry bits across the wire at such vastly different speeds are the different ways each of the 4 pairs of wires are used. We'll have to discuss three separate ideas: Standards and Wire Usage, Bits Transmitted, and Frequency.
Standards and Wire Usage
For example, 100BASE-TX is the predominant standard that governs transmission rates of 100mbps over UTP. It does this by dedicating one pair of wires for transmission, and the other pair of wires for receiving -- the remaining two pairs are unused.
1000BASE-T is the predominant standard that governs transmission rates of 1000mbps over UTP. It does this by using all four wire pairs, at the same time, in both directions. Each pair is responsible for carrying about 250Mbps of traffic at a time, providing for a total throughput of 1000Mbps, or 1Gbps.
One of the other differences between 100BASE-TX and 1000BASE-T is that each transmit bits across the wire differently.
Basically, at any given time a certain voltage signal on the wire represents a certain value. In 100BASE-TX, there are only two possible values that can exist: a value of
0 or a value of
1. You could say that at any given instance, a single bit can be transferred across the wire.
In 1000BASE-T, there are four possible voltage values that can exist on the wire:
11. Or, said differently, each instance of signal being applied to UTP in the 1000BASE-T standard transfers two bits at a time.
100BASE-TX calls for a frequency of 100Mhz, which means each a signal can be applied and read by the other side 100 million times per second. This ends up being the speed at which each pair of wires can transmit either a
1 or a
0. This is what gets 100BASE-TX its 100Mbps speed, because each 'instance' of signal being applied equates to a single bit being transmitted. And since there are two pairs being used (one in each direction), that equates to 100Mbps in one direction, and 100Mbps in the other, or a total of 100Mbps full duplex.
1000BASE-T calls for the use of 125Mhz frequency, which means a signal can be read off the (pair of) wires 125 million times per second. Since each instance of a signal being applied in 1000BASE-T sends two bits across the wire, this means each pair can transfer 250 million bits across the wire per second, or 250Mbps. Since there are four pairs being used, this grants a total of 1000Mbps, or 1gbps speed.
These are some of the methods that a single physical cabling specification (UTP) is able to transmit data at such vast different speeds (10Mbps, 100Mbps, 1000Mbps -- or 10/100/1000). Think of it from the terms of technology evolution -- engineers found new ways to transmit data over the same wire. As such, the single wire can do multiple speeds at the same time. A single UTP wire can do 10, 100, or 1000 million bits per second, hence the wires and interfaces are labeled 10/100/1000.
I'm afraid I don't know the specifics of how 10BASE-T transferred bits across the wire, so I can't describe how that works like I did the other two standards