Not eXactly C is a high-level programming language for the Lego Mindstorms NXT. NXC, which is short for Not eXactly C, is based on Next Byte Codes, an assembly language. NXC has a syntax like C.
You can download the german tutorial for free here: http://roberta-home.de/de/was-bietet-roberta/roberta-reihe/nxc-tutorial
The English original version is available here: http://bricxcc.sourceforge.net/nbc/nxcdoc/NXC_tutorial.pdf