The first thing to understand is that every point on a wave front acts like a pebble in a pond. When you drop a pebble in a pond it makes a circular ripple that moves outwards. When you get lots of them all in a row you see a long straight wave front. OK?
Diffraction - diffraction occurs when waves pass through a slit or around an edge. So, think of same water waves travelling through a slit, let's say coming through the Sydney heads. When it is a very wide slit (one that is bigger than the wavelength of the wave), you will see the wavefront move through the slit BUT at the edges it will be blocked by the two slit edges (ie the North and South Head). At this point the wave returns to looking like our pebble in a pond and the wave will spread out in a circular fashion. It will appear to sort of bend around into the shadow area behind the Heads.
This is how a region on the western side of Sri Lanka was affected by the Tsunami that came from Indonesia. The water waves diffracted around the back of the island.
When the slit is smaller than the wavelength of the wave, then all you see through the slit on the other side is the circular waves like that pebble in a pond.
Refraction - refraction is the actual bending of a wave when it travels from one region to another that has a different refractive index. So, when a water wave travels from deep water (which has a low refractive index) to shallow water (which has a high refractive index) the waves slow down, bunch up and the wavelength gets smaller. This is why light, when it passes through water seems to bend at the interface. Put a pen in a cup of water and look at it from the side. You are seeing refraction.
Dispersion - dispersion occurs when light is broken into its various wavelengths. By refracting the light the different wavelengths bend by a different amount. White light will spread out to look like a rainbow. This spreading out of the wavelengths is called dispersion.
I hope that helps you get a handle on it. It's kind of hard without pictures and stuff to show you.
