WHAT IS RESISTANCE?
In order for electrons to become an electrical current, they must move through the conductor down a potential gradient. However, the heat of the surroundings provides kinetic energy, causing them to vibrate in a random fashion. The effect of this is that instead of moving through the conductor like liquid flowing through a pipe, there are the electrical equivalent of small obstructions and eddies which prevent the "liquid" from flowing smoothly to its destination.

This has some interesting, and very useful, effects: probably the most important being resistance to current flow.

Resistance may be the result of physical collisions with other particles in the conductor, in which the electron will unavoidably lose some of its forward kinetic energy to the other particle. This heats up the conductor, causing the lattice to vibrate more and hence further collisions ensue. In addition, the lost energy of the electron must be regained in order for it to rejoin the flow of current through the conductor. This energy must come from the electric potential which would otherwise have served to project other electrons through the material.

Another source of resistance is long-distance electrostatic attraction or repulsion of the current-carrying electrons. Although the effects of this are not as pronounced as if a direct collision had occurred, they are exactly the same in effect. Furthermore, this interaction is far more common than physical collisions on account of the abundance of empty space within any material--although most electrons will pass through untouched, they will still be affected by the pervasive electromagnetic fields of the ions and other electrons.
Electron-Ion Interaction Diagrams
Image created by the author for this website

As can be seen from the diagrams above, the amount of energy used up in these interactions will be significant, and in a normal conductor proportional to the current flowing (i.e. the speed at which electrons travel through the material).

In a superconductor, however, this is not the case: under special conditions, there is no electrical resistance at all. The reasons for this and the effects it causes are covered in the next section.