JPG (also written JPEG) is the most widely used image format for photographs. It uses lossy compression, meaning it throws away some detail your eye is unlikely to notice in order to make files much smaller.
That trade-off is why JPG is perfect for photos on the web and for sharing: the files are small and open everywhere. The downside is that JPG doesn't support transparency, and saving the same JPG over and over slowly degrades quality.
You can convert to JPG, compress a JPG or turn JPGs into a PDF — all free and in your browser.