However, sometimes you know that you will need different, independantly adjustable resistors, or higher wattage (bigger, so they can carry more current and/or voltage), in these cases, you need to use discrete resistors.
Just to the right and below of the big black component with all the blue wires, is a blue object with stripes on it, which is a through hole or leaded resistor. It's hard to tell from this picture, but they actually sit up away from the board a little bit, rather than resting directly on the board like a surface mount component would. These resistors are the kind that were first used in most electronic designs. They are still used in some applications today, because they can be made bigger than SMT resistors, for higher wattage ratings. When acomponent uses a lots of power, which is measured in watts, it typically gives off a lot of heat. Since through hole resistors are bigger and therefore have more surface area to conduct away heat, they are much better at disipating heat than surface mount resitors. Also, since through hole resitors let air flow all the way around them, even more of the surface area is used for disipating heat.
The large black component in the middle is a 2.5 volt regulator. It puts out a very accurate and constant voltage, which can then be used as a reference. Both accuracy and consitency are important for reference voltages. The accuracy is needed so that when you make measurements with it, you will get the right answer, just like you need an accurate ruler to make good distance measurements. Similarly, you need your reference , or ruler, to be very consistent as time goes on, or else you could end up with a different result every time you measure something. That's why you don't see too many rubber rulers!