Why is silicon a poor electrical conductor and copper a good conductor?

It is due to the availablity of "free electrons". All metals such as Copper can be considered as atomic nuclei surrounded by a "sea" of electrons which are free to move around with little resistance, which makes all metals good conductors of electricity (and heat, incidentally). In Silicon, a semiconductor, the electrons are mostly locked up into the silicon crystal lattice structure and free electrons are rare, occurring only when thermal effects release them temporarily from the lattice (leaving behind what is called a "hole"). These free electrons and holes occurring due to thermal effects are called "intrinsic carriers" and the hotter the silicon gets, the more of these intrinsic carriers there will be around. Thus with silicon and other semiconductors, electric current flows more easily (less resistance) as the material gets hotter, while the reverse is true for metals.