Don't have any answers for you, but there are some excellent comments from a self post here on r/masseffect that touch on this: http://www.reddit.com/r/masseffect/comments/h4qts/what_are_the_reapers/

Specifically, octorocket and iandravid's comments feature some interesting info.

Octorocket's comment has some cut content from ME2:

It was lonely. It called to us.

It wanted to remember. The Masters had been gone so long.

The Masters were lost when it was shattered.

Currents swept through their inner worlds. They were turned to noise. Babble.

The worlds were empty. But the body lived. It lay fallow.

The heart pumped. The lungs breathed. But the mind forgot the Masters.

It called and They did not answer.

We have become an echo of Their echo.

We have become more than we were.

Join us. Know us. Remember all our lives.

We are no longer afraid.

You would never be lonely again.

We are not your enemy. We only wish to share ourselves.

We can join them. We can be like them.

We can reach the end of evolution.

Do not fear. It is wonderful to be us. We understand ourselves.

You cannot defeat them. They will lead us into eternity.

If you could only see how we see. Know what we've learned.

They were called imshai. Those who lived here before.

Reaper. One. A mechanical device used to cut ripened grain. Two. One who gathers a harvest.

Harvest. One. The consequence of an event or series of events. Two. The yield of a growing season. Three. To gather.

Shepard. They know you. They wish you to understand. They are shepherds, too.

And here's iandravid's comment, which has an illuminating conversation from Mass Effect: Retribution (SPOILER - do not read if you intend to read the books) between Kahlee Sanders and Paul Grayson (controlled by the Reapers):

K: Why are you here? What do you want from us?

P: We seek salvation, ours and yours.

K: Salvation? Is that what the Collectors were doing? Saving those human colonists? Is that what you did to Grayson?

P: He [Grayson] has been repurposed. He has been evolved to something greater than a random assortment of cells and organic refuse.

K: The randomness is what made him unique. it made him special. Why can't you just leave us alone? Why can't you let us live our lives in peace?

P: We are the keepers of the cycle. The creators & the destroyers. Your existence is a flicker, a spark. We can extinguish it - or we can preserve it. Submit to us and we can make you immortal.

K: I don't want to be immortal, I just want to be me.

P: Organic life lives, dies and is forgotten. You cannot fully comprehend anything beyond this. Yet there is a realm of existence beyond your understanding. We are the pinnacle of evolution. Yet we see potential in your species. You can be elevated. The weakness of organic flesh can be cast aside. You can transcend yourselves. Your knowledge is limited by genetics. You cannot see beyond the brief instant of your own existence. Yet our knowledge is infinite, as are we. The laws of this universe are inviolable, immutable. Your resistance will only lead to your extinction. What we do is inevitable.

*edit: formatting