Well, to understand their difference lets figure out how these two metals are made.
14k Gold Plated Jewelry
During the process of gold plating a thin layer of gold is applied to a base metal either copper, nickel, silver and or brass via electrochemical plating.
Long story short… 14k gold plated jewelry is like giving base metal a bath in a chemical solution that includes a concentrated form of 14k gold.
Next, a current of electricity is administered to the bath which results in the base metal being coated with a microscopic film / VERY thin layer of 14k gold.
Due to the VERY thin layer of gold on plated jewelry in time your 14k gold plated jewelry will fade, wear off, tarnish and or chip.
14k Gold Filled Jewelry
14k gold filled metal is created by mechanically bonding a layer of 14k gold by heat and pressure onto an alloy/base metal core.
An actual layer of Karat gold covers the entire alloy/base metal surface.
This mechanical bonding process is permanent thus making 14k gold filled jewelry tarnish resistant.
The value of gold-filled metal is greater than gold-plated metal because gold-filled has an actual layer of karat gold, not just a microscopic film that 14k gold plated metals have.
I hope this information was helpful! If you have questions just ask :)