Furnace is essentially a framework to make it really simple to build event processing pipelines. To be able to construct such a system, you need a few different things. Thinking left to right, input to output:
Inputs: to be able to ingest data into it. Everything else wouldn't be very useful otherwise. We think plumbing provides a great analogy to many of the terms an event processing system uses, so in Furnace, on the input side we have sources and taps.
Processing bits: to act on the ingested data and either enrich it, modify it or do any sort of work with it. We call them functions
Outputs: to send that data somewhere else, whether it is a database, cold storage or another system. Following the plumbing analogy, we have called them sinks.
With those three sections you have the basic building blocks ready and you just need to piece them together and be sure that it's easy to do that.