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.
How JPG compression works
JPG doesn't store your photo pixel by pixel. It breaks the image into 8x8 blocks and describes each one as a combination of frequency patterns, then discards the high-frequency components your eye is least sensitive to. The quality slider controls how much gets discarded. This is why JPG excels at photographs, where colour changes gradually, and struggles with screenshots and text, where every sharp edge is a high-frequency detail it wants to throw away.
Generation loss
Because each save re-runs that process on an already-degraded image, editing and re-saving the same JPG repeatedly compounds the damage. Edges soften, blocky artefacts creep in around text, and flat areas develop faint blotches. Opening a JPG and closing it without saving costs nothing — it is the re-encoding that hurts. If an image will go through several rounds of editing, work in PNG and export to JPG once at the end.
When not to use JPG
Use PNG or WebP instead when the image has a transparent background, when it is a screenshot, diagram or line art with hard edges, or when it will be edited further. JPG has no transparency at all, so a logo saved as JPG gains a solid background — usually white — that cannot be removed afterwards without cutting it out again.
JPG vs JPEG
They are the same format. The difference is historical: early Windows limited file extensions to three characters, so JPEG became JPG. Both open identically everywhere and there is no quality difference between them.