Stanton's answer is a good one, so I'm not going to repeat it here (read it, upvote it, etc.) But there is something I would like to add, namely, where to mitochondria come from?
Now, eukaryotes are highly reliant on mitochondria in order to metabolize food. But a curious thing about mitochondria is that they have their own DNA, independent of the nucleus and in fact, not derived from the nucleus. Why's that? The current theory is that mitochondria were once separate prokaryotic organisms that were engulfed by early eukaryotic cells. The eukaryotic cell that entered into a symbiotic relationship with those engulfed mitochondria had a huge evolutionary advantage: now they could make a lot more energy. And from the perspective of the mitochondrion, if you have a cell that's protecting and feeding you, well, that's pretty cool, so you're happy with the arrangement.
In short, every eukaryotic cell pretty much contains what used to be prokaryotic cells inside of it.