The U.S. National Lightning Detection Network (NLDN) is a resource that has a wide array of applications, including many meteorological applications. Our objective in this paper is to foster a better understanding of the data produced by the NLDN so that end users of the data sets can make the most effective use of the data. To that end, this paper starts with a discussion of how the NLDN detects lightning. This is followed by a discussion of the data sets available from the NLDN. The NLDN detects primarily cloud-to-ground (CG) lightning, and the most widely used data products are the CG stroke and CG flash data sets. However, the network is also capable of detecting a small fraction of cloud lightning as well, and a cloud discharge data set is also available. The NLDN sensors are also capable of detecting signals from distant lightning outside of the continental U.S. By processing the data in a way that permits these long-distance detections, we produce an additional long-range data set. The NLDN data sets are subject to continuous quality control. Some of the quality control parameters are made available with the data sets, and we describe those in this paper. Finally, independent validation studies are regularly carried out to measure the detection efficiency and location accuracy of the network. The most recent studies of this kind are summarized in this paper.