The Portable Network Graphic, or PNG, is the most versatile of the Web graphic formats. However, not all Web browsers can take full advantage of PNG characteristics without using plug-ins. A PNG can support up to 32-bit color, can contain transparency or an alpha channel, and can be progressive.
PNG compression is lossless, even in high color depths. It compresses across rows and columns of pixels, often yielding better compression than GIF, which only scans rows.
The PNG format is best for creating complex live transparency, high-color graphics, and better compressed low-color graphics.
PNG is the native file format for Fireworks. However, Fireworks PNG files contain additional source information that is not saved when you export a PNG for use on the Web.