I might switch the two around so forgive me if that's the case, but here goes:
dissipation - tells how quickly the property dies. After it's set to 100 in a voxel, how quicly will it dissipate to 0 if nothing else influences the property in that voxel - only the time is the variable.
Ofcourse it will never dissipate to 0 completely, thus you have the minimum threshold parameter below which the algorithm considers the voxel clear of that parameter (smoke or temperature). Higher dissipation - property dies faster.
diffussion - says how much the property spreads to neighbouring voxels. In real life if you pop a balloon full of smoke, it won't stay in the same place - it will diffuse around the volume getting bigger (more spread) and thinner (lower density)
The higher the diffusion - the faster the property will spread around.
As you can see they are also connected - if you set a high diffusion, the smoke will also dissipate faster - not because the time has gone, but because the smoke particles have left the voxel and went to another one.
For smoke it's pretty self explanatory - you set how longlasting (lifespan) you want the smoke to be and how well keeping-together.
For temperature, since you can't see it, the effect is a little more complicated.
For example, if ou want to make a fuel cloud ignite from a single place and then have the flame spread around the fuel cloud, you have to set a low dissipation, so that your temperature doesn't die to quickly and high diffusion so that the temperature will easily travel to neighbouring voxels and ignite them.
