Hi, I have read a lot about Diffusion Equation and solving neutron flux problems in different mediums, planes and groups, but I can't grasp this topic. In other words, I don't know why they mention: 1. Infinite/finite medium 2. Homogeneous/non-Homogenous medium 3. One/two or multi-group (what do they mean by group?) I would appreciate it if someone could explain this topic with analogies. Thank you.