An effect created by juxtaposing two contrasting or contradictory ideas. John wanted to try to pull the man out of the burning car. I told him it was too late, that we needed to run for help, first. The main idea, the reason you're writing, the backbone of the essay. The aspect of text that conveys a writer's attitude toward the subject or the audience.
Tone can be ironic, sarcastic, nostalgic, informative, plaintive, and so on.
Unity in a paragraph implies a sustained purpose, and forbids digressions and irrelevant matter. The way elements like word choice and tone add up to create an effect on the reader. Different voices create different effects on the reader. An angry voice might make the reader defensive. A sad voice might make the reader feel sad, too.