Distributed computing or grid computing is the practice of dividing up a very complex problem into many parts then each part goes to one computer. This is an alternative to using one super powerful computer to do the entire problem by itself. It can be as simple as 2 pc's or multiple PC's in a network or you can have millions of Pc's connected by the internet.
I used to do it. I was a member of seti@home for a long time. What made me stop - cool n quiet and rising electricity bills.
One day I bought a new PC that was based on the AMD athlon XP processor. It had cool n quiet. Cool n quiet underclocks your CPU when your PC is doing simple tasks that dont stress out the CPU. This reduces heat and power consumption. I used to run seti@home in the background (low priority to avoid slowing down my PC) whenever I was using my pc for surfing or typing. I dont leave my PC on 24/7 or overnight because I dont want a high electricity bill and I dont leave my PC on when I'm not home because of the fire risk. But with cool n quiet, I suddenly had a big incentive to not use SETI@home because it would slightly lower my electricity bill.
Nowadays, with electricity prices going up and some form of power saving standard on all CPU's, there is little hope for distributed computing in my house. Yeah I know, its selfish, but thats the way it is.
Again, this is MY personal reason. But I'm sure other people share my sentiments.