A photo or screenshot saved as PNG can easily be 5 to 10 times larger than it needs to be for email, a web upload or a form with a size limit. JPG (also written JPEG) brings that down at the cost of exactness: it is lossy, and it has no transparency, so any see-through area has to be filled with a solid colour. Crisp text and flat colour can pick up faint artefacts around edges; for those, staying with PNG or choosing WebP gives a cleaner result.
JPG has no transparency, so see-through areas of a PNG are filled with a solid colour, white by default; you can change it under Options. The defaults suit most files. Under Options you can set the quality, aim for a target file size and resize the result.
Camera metadata such as EXIF and GPS location is not copied into the JPG, and the photo's rotation is applied so it stays upright.