Truly the heart of a large number of Guitar players sound, effect pedals are a great addition to any instrument with a pickup.
There are a few different main types.
Firstly, there are distortion type pedals (distortion, fuzz, and overdrive). Whilst its a bit too much to go in to here, distortion is basically amplifying and compressing the audio, making the sound become louder, larger, and dirtier. Distortion is the most common type of guitar effect.
Next, there are modulation type pedals (flanger, chorus, phaser). These pedals add colour to your sound, either by changing the phase or by doubling your sound. The best way to explain these sounds is by listening to some audio examples, like these (youtube links)
Next, you have time effect pedals (delay and reverb). Basically, a delay takes your sound, records it, and plays it later. This can be anywhere from .001 of a second later to 4 seconds later, depending on the settings and pedal type. Reverb is basically like an echo for your guitar, adding depth to the sound.
Lastly, the other pedals you'll find here are important ones like tuner pedals, which are a tuning setup on the floor, compression and dynamics pedals, which are great for achieving more sustain or outputting a more balanced sound, D.I boxes for live sound, and a host of other items.