What is PNG?
PNG is the web's lossless image format: compression that never degrades the picture, and a real alpha channel for transparency. It is the right choice for screenshots, logos, line art and anything with sharp edges or text.
What is TGA?
Truevision TGA is a fixture of game development and 3D texturing, where its straightforward layout and real alpha channel have kept it in use long after browsers stopped caring about it. Files may be raw or run-length encoded.