It is unclear what different chains mean in configuration

### What was wrong?

Different chains are listed and present by `populus.json` as documented here:

http://populus.readthedocs.io/en/v1.5.0/config.html

However the documentation does not give any background what are these chains and what is their purpose.

#### Cute Animal Picture

```
,'.-.'.
'\~ o/` ,,
{ @ } f
/`-'\$
(_/-\_)
```