JPG (also written JPEG) is the most widely used photo format on the web. The format dates to 1992 and uses lossy compression, discarding image detail in exchange for dramatically smaller files. It can't carry transparency. Modern alternatives like WebP and AVIF compress 25-50% better at the same visual quality, but JPG remains the universal compatibility default: every browser, OS, and image editor in existence reads it.
How to open
Every operating system opens JPG natively, double-click on Windows, macOS, Linux, iOS, or Android and the system viewer launches. Browsers render JPGs inline. Image editors (Photoshop, GIMP, Affinity Photo, Pixelmator, Photopea in the browser) all read and write JPG.