NaCl (pronounced "salt") is a new easy-to-use high-speed software library for network communication, encryption, decryption, signatures, etc. NaCl's goal is to provide all of the core operations needed to build higher-level cryptographic tools.
Of course, other libraries already exist for these core operations. NaCl advances the state of the art by improving security, by improving usability, and by improving speed.
The following report contrasts NaCl with other libraries from a security perspective: (PDF) Daniel J. Bernstein, Tanja Lange, Peter Schwabe, "The security impact of a new cryptographic library". Pages 159–176 in Proceedings of LatinCrypt 2012, edited by Alejandro Hevia and Gregory Neven, Lecture Notes in Computer Science 7533, Springer, 2012. ISBN 978-3-642-33480-1.
The following report was created for Research Plaza and gives an introduction to NaCl for a wider audience: (PDF)