A Lightening Talk is a shot 3-10 minute talk, which anyone can propose, where the timing is strictly adhered to by a moderator. It is a way of getting many community contributions to a conference in a short space of time without a lot of administration.
YOUTUBE jmY9VRq5e1Y Lightning Talks Day 3 [31c3]. You can see the on screen 3 minute progress warning, which at [31c3] is also augmented by a physical LED light that promotes audience feedback with regard to the timing.
Most conferences will allot a segment of roughly 30 to 90 minutes long to speakers. Talks are arranged one after the other during the sessions. The talks are usually given at conferences in order for the event to have many speakers discuss a multitude of topics.
Goal
The goal of lightning talks is to articulate a topic in a quick, insightful, and clear manner. These concise and efficient talks are intended to grab the attention of the audience, convey key information, and allow for several presenters to share their ideas in a brief period of time - wikipedia 
Format
The format requires the speaker to make his or her point clearly and rid the presentation of non-critical information. This causes the audience to be more attentive to the speaker and gain a broader array of knowledge from the presentations given.
The format of lightning talks varies greatly from conference to conference. Slides may be discouraged, and a single computer running a Presentation program is used by all speakers. In general lighting talks are given in a format that can include slides but if so, the speaker must be careful not to read the details which they include.
The term Data Blitz is sometimes used to refer to a session of lightning talks, particularly at academic conferences in the sciences and social sciences such as the annual conference of the Society of Personality and Social Psychology.