JPG and PNG are the two most common image formats on the web, and people constantly need to switch between them. Here's the honest, no-jargon difference, and how to convert either way in seconds.
JPG uses "lossy" compression: it throws away detail your eye is unlikely to notice to make the file much smaller. That's perfect for photos, where tiny imperfections don't matter. The downsides: it can't store a transparent background, and sharp edges (like black text on white) can get fuzzy "artifacts".
PNG uses "lossless" compression: nothing is thrown away, so lines and text stay perfectly sharp, and it supports transparency. The trade-off is bigger files, a photo saved as PNG can be several times larger than the same photo as JPG.
If your goal is simply a smaller file, also try the Compress tool, which lets you target an exact size in KB. For most photos, JPG at 70–80% quality is the sweet spot between looks and size.
WebP is a newer web format that gives smaller files than JPG or PNG and works in all modern browsers. HEIC is what recent iPhones use by default. It saves space on the phone, but many websites and older programs cannot open it, so convert HEIC photos to JPG before uploading them to forms or sending them to other people.
JPG compresses in small blocks and blurs hard edges slightly. Black text on white is the worst case, which is why screenshots of documents look fuzzy as JPG. Save screenshots, charts and anything with text as PNG, and keep JPG for photographs.
The shape stays the same, but any transparent area becomes solid white and edges can show faint halos. If your logo needs to sit on a coloured background, keep the PNG. Only use a JPG version where a plain white background is acceptable.
For print, quality matters more than file size. Use the largest original you have. A high quality JPG is fine for photographs, and PNG or PDF is better for line art and text. Resolution (pixels) matters more than the format itself, so avoid enlarging small images.
No. Converting cannot bring back detail that was already lost. A JPG saved as PNG will look identical but the file will be much bigger. The only benefits are being able to edit without further loss and meeting an upload rule that demands PNG.