What is the difference between single - core and multi - core?

Single conductor :

There is only one conductor in the insulating layer of single core wire. Advantages:

strong tensile resistance, not easy to be broken by mold, convenient threading and good stability. Disadvantages: poor heat dissipation, poor softness and poor bending resistance. 

Multiconductor :

There are two or more conductors in the insulation layer of multi-core wire, advantages:

good softness, good heat dissipation, good skin resistance, strong load capacity, disadvantages: poor tensile resistance, easy to moldy break, joint should be handled well.