What are developer products and how do i make one?

Along with the Wiki post, here's an example.

h.ClickDetector.MouseClick:connect(function(p) -- I happened to have my event fire from a ClickDetector
		game:GetService("MarketplaceService"):PromptProductPurchase(p, id) -- Make them purchase it.
		-- setup local variables
local MarketplaceService = Game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")
MarketplaceService.ProcessReceipt = function(receiptInfo) -- Basically, a digital reciept
		if p.userId == receiptInfo.PlayerId then
			if receiptInfo.ProductId == id then
				p.Character.Humanoid.Health = p.Charaacter.Humanoid.Health + 150
		end
	end	
	
	-- record the transaction in a Data Store
	local playerProductKey = receiptInfo.PlayerId .. "_got_" .. receiptInfo.PurchaseId -- Logs it in Data Store
	ds:IncrementAsync(playerProductKey, 1)	
	
	-- tell ROBLOX that we have successfully handled the transaction
	return Enum.ProductPurchaseDecision.PurchaseGranted		
	
	end
end)

Developer products can be purchased multiple times by anyone. Yes, that includes the owner of the game as well. There's many uses for them such as buying an in-game house, buying in-game currency, etc. The possibilities are limitless.