What is the difference between ethernet cable and regular phone line cable?

There are many different types of Ethernet cables.  Most modern Ethernet cables are one of the following:  Category 5e, Category 6, Category 6a, Category 7, and Category 7a.  Categories lower than 5e have become obsolete.  The main difference between these cable categories is the bandwidth (bandwidth can be thought of as speed although technically it is different) of data that can travel across them.

There are distance limits on cables too.  Basically most Ethernet standards recommend not going over 100 meters in length unless you are using fiber.

Category (CAT #x) cables have 8 copper wires in them which are twisted into pairs for more efficiency.   Cables can be shielded or unshielded, shielded cables resist outside interference better but can cost a lot more and are less flexible physically. 

CAT cables also have a connector on the end called RJ45.  CAT cables are normally used to interconnect computers and Internet Protocol (IP) networks.

This is different from the phone cables (although this is changing with the advent of Digital Subscriber Lines (DSL).

Regular phone cable is normally not used for interconnecting computer networks, DSL withstanding.   For the most part it is considered obsolete so I will not dwell on details, but it too has 8 (older wiring may have only 4) wires but normally only 4 are used.

Regular phone cable has a connector on the end called RJ11, which is smaller than a CAT cable connector (RJ45).  So you can easily tell the difference between CAT and Plain Old Telephone Service (POTS) cable by the connector.  POTS connectors are half the size of CAT connectors.  Again there are exceptions but this is generally correct for most cases.

Phone cables are normally only used to connect home phone service and DSL.