One input is processed and then written to one output. This includes effect processing, normal sample playback, format conversions, etc.
Lots of inputs are mixed into one output.
There's at least one realtime input and one realtime output. Signal is sampled from the realtime input, processed and written to the realtime output.
One input is processed and written to one or more outputs.
The most common situation is that there are two separate chains. First one consists of realtime input routed to a non-realtime output. This is the recording chain. The other one is the monitor chain and it consists of one or more non-realtime inputs routed to a realtime output. You could also route your realtime input to the monitoring chain, but this is not recommended because of severe timing problems. To synchronize these two separate chains, ecasound uses a special multitrack mode (which should be enabled automatically).
Just like multirack recording. The only difference is that realtime input and output are externally connected.