A Category 5 Ethernet cable (a.k.a. Cat 5 or RJ45) has eight wires in the end whereas a typical phone cable (RJ11) only has two wires in the end.

NOTE: Some phone cables may have four wires in the end as well, but these are less common.

Most phone cables are grey, white, or black but the color can vary.

Typically, Ethernet cables are blue, white, or red; but they can be different colors, as well.

The phone cable will fit into both the phone jack and the data jack on the wall, but an Ethernet cable will only fit in the data jack.

The wire portion of an Ethernet cable is thicker than that of a phone cable, and phone cables are normally flat while Ethernet cables are rounded.

The connector end of an Ethernet cable is about twice as long as that of a phone cable

NOTE: There is a difference between a normal Ethernet cable and a crossover cable. A normal Ethernet cable will have the same wiring order on both ends of the cable. A crossover cable will have different wiring order on either end of the cable.