There are basically two different types of guitar and bass effects, the traditional type - and the one that has been around for the past 40 years is the single effect pedal whilst the secondary type is the more recent multi-effects processor.
Single Effect Foot Pedals
Essentially the single pedal is a foot pedal that produces a single effect or in a few cases a small number of effects, and until the recent invention of the micro-chip processor was the most popular type of effect pedal.
The way the pedal works is by processing the signal from the guitar with two jack cables required to complete the connectivity. One cable connects to the amplifier with the other end plugged into the corresponding "Amplifier" interface on the pedal. The other jack cable connects directly to the guitar with the other end plugged into the "Instrument" interface on the pedal itself. In this way the signal is routed through the pedal and back to the amplifier.
Linking Numerous Effects
Today, most pedals have an LED light that illuminates when the pedal is activated to provide the user with a visual indication that the effect is engaged.
It is also very common for guitarists to link together several pedals at once by using short patch leads to "daisy chain" the pedals together.