Shift registers are first cheep choice when you need to expand digital I/O pins say from one to eight. All you need is to clock serial data in in order to get all bits paralleled. Mika gives couple practical examples how this can be done using simple bit banging or SPI bus interface.
As microcontroller he have chosen Teensy board. As programming tool he selected Arduino IDE with Teensyduino add-on. In programming examples he used two types of shift registers – SN74HC595 and TPIC6B595. Second one is used in higher power applications. Both of them need three pins on input side: Data, Latch and Clock. As mentioned above it can be done using bit bang method where data mus be clocked in in program flow. So you need to write function that takes care of clock signal, latch and data output. This method is more obvious but occupies processor time. While using built in SPI you can set and forget. Data is shifted out automatically without using processor time.