PNG (Portable Network Graphics) is a lossless image format that supports full alpha channel transparency. Unlike JPEG, compressing and decompressing a PNG produces a bit-for-bit identical copy of the original — no quality loss whatsoever. PNG was designed as an open alternative to GIF and is now the standard for web graphics that aren't photographs.
PNG uses a combination of delta filtering (storing pixel differences rather than absolute values) and DEFLATE compression (a lossless algorithm similar to ZIP). This works extremely well for images with large areas of flat color — logos, screenshots, UI elements — but produces larger files than JPEG for complex photographs. A photo saved as PNG vs JPEG at 80% can be 3-5x larger.
PNG supports three color modes: grayscale, RGB, and RGBA (with alpha transparency). PNG-8 (indexed color, 256 colors max) is useful for simple graphics, while PNG-24 and PNG-32 support full color and transparency respectively. For web images that need transparency, PNG is the standard choice — WebP lossless is a modern alternative with better compression.