TGA, or TARGA, is a bitmap format officially known as Truevision Advanced Raster Graphics Adapter. The format was developed by Truevision, Inc. which has since been acquired by Avid Technology. The format was designed for TARGA and VISTA graphic cards found in early PCs.
The graphic format supports 8, 15, 16, 24, or 32 bit color and RLE compression. PCs tend to feature the *.tga extension, while Macintosh uses a TPIC type code.
Technical specifications can be found here: http://paulbourke.net/dataformats/tga/