Ethernet cables are the cables used to connect your computer to the router/switch and the DSL/cable modem. the are two common cable and they are straight thru and cross-over. Each is wired differently and is used in different situations. Sometimes you will see a straight-thru cable called an Ethernet cable.
Both cables are made of twisted pair cables that have 4 pairs of wires in them. Usually you will see it called Cat 5 wire or the latest may use Cat 6 wire. The connectors on the end look like telephone plugs but are bigger. The phone plug only uses 2 or 3 pairs and is called a RJ-11 plug. The network cable uses 4 pairs and is called a RJ-45 plug. Basically the cables can be up to 100 meters (300 feet or length of football field).
The straight thru cable is used to connect different type devices together. It is used to connect a computer to the router/switch and to connect the router/switch to the modem. The wires in the plugs on each end are in the same position. If you held the plugs by each other you would see the orange in the same location on both ends and the green in the same position and the blue and brown pairs. There is a specific order the colors are supposed to be in, but some commercial patch cables do not follow it.
The cross-over cable is used to connect two of the same type devices. If you wanted to directly hook to computers together you would use a cross-over cable. Basically the crossover cable switches the pair on one end that is used to send information to the pins on the other end that receives information. If you had tried to use a straight thru basically both machines would listen on the same pair and talk on one pair. That would not work. So the receive and transmit pairs cross in the cable and attach ion the opposite at the other end. Computers and router/switches use different pairs to receive and send so they can use a straight-thru cable.
Most of the time you will use straight thru cables which is what comes with router/switches and cable/DSL modems.