Lightning Talks are five-minute talks in a ninety-minute or sixty-minute time slot.
Maybe you've never given a talk before, and you'd like to start small. For a Lightning Talk, you don't need to make slides, and if you do decide to make slides, you only need to make three.
Maybe you're nervous and you're afraid you'll mess up. It's a lot easier to plan and deliver a five minute talk than it is to deliver a long talk. And if you do mess up, at least the painful part will be over quickly.
Maybe you don't have much to say. Maybe you just want to ask a question, or invite people to help you with your project, or boast about something you did, or tell a short cautionary story. These things are all interesting and worth talking about, but there might not be enough to say about them to fill up thirty minutes.
Maybe you have a lot of things to say, and you're already going to give a long talk on one of them, and you don't want to hog the spotlight. There's nothing wrong with giving several Lightning Talks. Hey, they're only five minutes.
On the other side, people might want to come to a lightning talk when they wouldn't come to a long talk on the same subject. The risk for the attendees is smaller: If the talk turns out to be dull, or if the person giving the talk turns out to be a really bad speaker, well, at least it's over in five minutes. With lightning talks, you're never stuck in some boring lecture for forty-five minutes.
A lot of people are not getting the point of these. They hear that they only get five minutes to speak, and they get all tense. "How can I squeeze a whole talk into five minutes?"
No! The point is that because the talk is only five minutes long, you don't have to take it so seriously. Just try to say something brief and interesting, and then get out in a hurry. If people want to follow up or ask questions, they will catch you in the hallway afterwards.
If I got to schedule eleven talks that were nothing but five minutes of ranting and raving, I would be really happy.