Pedals are for adding more varieties to your sound. The differences are the sounds they make. A delay would cause it to be like an echo effect, where you strike a note, and it repeats while fading away. Flanger would be like a jet flying overhead, but in your amp. There a tons out there, you just gotta get out there and try them.

and you know which one you want if it adds that one aspect to the sound and tone you're looking for.
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Pedals are portable, gig-worthy effects units that effect your signal in some way. They are ususally more noisy and of lower tonal quality than rack-mounted effects units found in studios, but if you want a given effect at a gig, no one in the audiance will care anyway.

There are dozens of different ways to modify a signal: flanging, compression, phase shifting, echo, reverb, chorusing, doubling, pitch shifting, distortions, etc. I find that a good reverb unit and a good chorusing unit are all you really need once you get serious (and mabye a distortion or two if you don't have a tube amp or a distortion channel on your amp); phasing and flanging and drop octave with metal distortion is fun, but rather hard to find something to do with.