Making it really simple...
Fe seeks external harmony, which can work it's way inward.
Fi seeks internal harmony, which can work it's way outward.
They kind of have similar goals, but work from opposite ends and with different methods.
Making it long-winded...
Fi is very imaginative, and receives it's ideas from some innate, unconscious place. It evaluates these ideas, or "feelings", and works them into a value system so that they all fit together like a puzzle and form a big picture. Conflicting ideas and emotions is often the cause of the internal turmoil many INFPs feel. We need to be peaceful inside, so sorting this out is a primary concern.
An INFP can apply this system externally through Ne, which makes universal application in a metaphorical manner, meaning the Fi-dom does not have to experience something to be able to understand it and how it may make someone feel. This is where our ability to empathize comes from.
Fe adjusts itself to the external, to promote that outer peace. As we all know, when the external is harmonious, then we're more at ease as individuals, and so it seeps its way inward. Fe can be a sort of social lubricant.
Evaluations are made by a general consensus, in that Fe does not assign it's own subjective value, as Fi does. Fe may not trust it's own judgment, just as Fi may be suspicious of what is generally accepted. Fe has an innate understanding of how these standards work, and sees the value behind them. In a sense, Fe creates the external standards by understanding them to promote them as principles and not just arbitrary standards.
Since Ni does not evaluate internally, the ideas and perceptions it makes are evaluated based upon the external. For an INFJ, that means the value is assigned through Fe. I've heard INFJs say they know what is right for someone else more easily than knowing their own feelings. That's hard for me to grasp.
I imagine that Ni might be a Fe bullshit detector, but Fe people can tell me if that is true or not. I suppose that involves evaluation, but a Ni image might inspire Fe to re-evaluate the external values.
I could blather on, but I think that's enough.