The difference between a flue and a chimney? If you imagime a polo mint then the mint is the chimney and the hole is the flue.
Each fireplace will have its own flue (fireplaces do not share) but these flues may all end up in the same chimney (that’s when you see two or more chimney pots in a row). If flues run close together then the bricks or stone seperating them are called the “feathers”. Feathers can develop holes (e.g. mortar crumbles and falls out or a brick falls out). Then smoke can jump across from one flue to another.
When you look at key building regs you will see that the minimum recommended height for a chimney is 4.5m (above the appliance).
A chimney can be made from a variety of materials, usually brick, stone, block, concrete or stainless steel.
“The combustion flue gases inside the chimneys or stacks are much hotter than the ambient outside air and therefore less dense than the ambient air. That causes the bottom of the vertical column of hot flue gas to have a lower pressure than the pressure at the bottom of a corresponding column of outside air. That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney.” http://en.wikipedia.org/wiki/Chimney