aftcast is a programmable variably-stochastic abstract image generator. The user is presented with a grid-based canvas which they can divide into zones. The user can then specify the probability that any block in that zone will be filled and with what colour to fill them. The resulting code produces a semi-random image which can then be shared with other people.
To put it less prosaically: imagine you're on the third floor of a building, standing out on the balcony. With you are some large buckets, numbered '1', '2', and '3', full of coloured foam blocks. The first bucket is nearly completely full of these blocks, while the second and third buckets are only half-full. The blocks in bucket one are painted shades of grey, while in bucket two they are red and in bucket three they are blue.
On the ground you have set up some temporary fencing, dividing the pavement into three zones. In the centre of each zone is a number written in chalk: '1', '2', or '3'.
You pick up bucket number 1, walk over to the edge of the balcony, and pour its contents out onto zone 1 of the pavement. Grey foam blocks scatter randomly on the ground; zone one has been almost completely covered with them.
You repeat this process for the two remaining buckets, pouring the contents of bucket 2 over zone 2 and bucket 3 over zone 3. Since these buckets weren't as full as the first, the blocks that have settled on zones two and three are packed less densely.
Gazing down over the balcony at what you've just accomplished, you announce to the world that your latest abstract art piece is finished. No-one really cares, but as an abstract artist you are no doubt used to such a response.
Two weeks later, you find yourself on the third floor of another building, with three buckets filled with foam blocks, just like last time. The pavement has been divided into three zones, just like last time. You pour each bucket into the corresponding zone, just like last time, and announce the completion of your latest piece of abstract art to a lukewarm reception, just like last time.
It's not exactly like last time, though. It could never be, no matter how many times you repeat the procedure, because the foam blocks are always going to be randomly scattered all over the place. Each zone will have approximately the same number of blocks in it, but the specific placement of the blocks will always be different.
Now replace the building and pavement with a website and the buckets and blocks with code. That's aftcast.