LEGO Mindstorms NXT : Programming

Very simple programs can be written using the menu on the NXT. More complicated programs and sound files can be downloaded using a USB port or wirelessly using Bluetooth. Files can also be copied between two NXT bricks wirelessly, and some mobile phones can be used as a remote control. Up to three NXT bricks can communicate simultaneously via Bluetooth when user created programs are run.
The retail version of the kit includes software for writing programs that runs on PC and Macintosh personal computers. The software is based on National Instruments LabView and provides a visual programming language for writing simple programs and downloading them to the NXT Brick.

NXT-G

NXT-G v1.0 is the programming software that comes bundled with the NXT. There are two different programming interfaces. One is included with the retail and educational kits and the other can be purchased separately. This software is adequate for basic programming, like to drive motors, incorporate sensor inputs, do calculations, learn simplified programming structures and flow control, etc. There are several pros and cons to using version 1.0 of this software, some of which are listed here:

Pros:
Easy installation on Windows XP and Vista machines as well as Mac OS X machines.
Can transfer data via Bluetooth or included USB cable.
Easy to Use, drag and drop, graphical environment.
Data wires show data flow from block to block.

Cons:
The programs executed on the NXT can be much larger than when compiled by another third party programming language. (e.g. 12kb versus 2kb)
Programs take substantially longer to load than other third party programs.
When creating large programs, NXT-G tends to have errors that make it crash, and lose unsaved data.
NXT-G software usually runs sluggishly, even on powerful PCs.
Most of these issues have been addressed in the recently-released NXT-G v1.1 version of the software.

Next Byte Codes & Not eXactly C

Next Byte Codes (NBC) is a simple language with an assembly language syntax that can be used to program the NXT brick.
Not eXactly C (NXC) is a high level language, similar to C, built on top of the NBC compiler. It can also be used to program the NXT brick. NXC is basically NQC for the NXT.

leJOS NXJ

leJOS NXJ is a high level language based on Java that requires custom firmware.