hi
Try thinking in terms of low-current signals and high-current signals.
Low current signals like the output pins of the Arduino or a 595 shift register can drive a limited amount of current-- about 20-30ma, or one LED.
That same pin can be fed into a transistor amplifier that amplifies the current-carrying capacity of the low-level signal. The high-current signal that results can drive 2,4, 10, 20 or 100 leds, depending on the type of transistor used.
D