CD4021 is a cascadable 8bit parallel to serial shift register (basically the inverse of the 74hc595).
Sometimes you'll end up needing more digital input than the pins on your microcontroller board can readily handle. Using a parallel to serial shift register allows you collect information from 8 or more switches while only using 3 of the pins on your...
The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element. Derivatives provide up to four timing circuits in one package.
The very popular 595 shift register chip of the 74 series is used a lot by hobbyis because of its wide availability, good speed and simplicity. Just plug it on an arduino like shown in the tutorials and youi have 8 more outputs, put two and you have 16 more outputs ! (up to a lot of them actually, with only three pins !)