In EVE, there are a ton of Isk faucets from which to drink. True faucets like Mining, Killing Red Crosses (bounties, reward, and loot), Exploration, PI extraction, Moon Mining, Gas Harvesting, R&D Agents all introduce things into the game where there was nothing before. Buying PLEX's also qualifies here, but is a different faucet all together.
"Secondary" ways to make Isk like production, BP market, Invention and reactions (moon goo and PI) add value to something and allow profit (sometimes), but does not introduce things into the game. It messes with the Isk in the basin, but does not create "something out of nothing".
A last category allows individuals to make a profit, and really doesn't create anything new. Trading, Scamming, PvP looting, etc all can be great ways to make Isk, but they are more a distribution of Isk and certainly does not add anything to the game (and here I mean item wise, definitely not game content wise, as all of these are fascinating aspects of the game)
As for the Isk sinks, they can be easy to understand (like NPC Corp Taxes, Taxes and fees in general, ship loss), or hard to understand (like Null Sov costs, Office Rentals, Slot fees, POS Fuel, PLEX for Account time).
At the end of the day, Isk in EVE flows from faucets to sinks. We just try to tap into our share before it disappears down the drain.
Individuals have their own personal faucets and sinks in their wallet. This carries over to corporations and alliances as well.
This never-ending flow of Isk is one of the most intriguing aspects of the game. I am always amazed at how people make their Isk, and also how the spend or lose it. People spend hours aiming their mining lasers at asteroids, make a ton of Isk, and then buy a fail fit Hulk and lose it to gankers. Others spend hours running missions, ratting in belts and do everything they can to get the Isk together to buy a 17B ship that needs a second account to offline it, just to say they have it. Others detest all the ways to make Isk so much they just fund their accounts with PLEX. Nothing wrong with that if you can afford it.
Personally, I spend almost as much time tweaking my "business" as I do thinking of fittings and tactics and other things I do for PvP fun. My wife has learned not to ask what I am doing when I have my binder out and I am sitting on the couch madly pounding numbers into the calculator. She just rolls her eyes and mutters something about "that stupid game".
Oh, if she only really understood.