HEIC (based on the HEIF standard) is the image format Apple devices use by default. It stores photos at roughly half the file size of a JPG at the same quality, saving space on your phone.
The catch is that many non-Apple apps, websites and older programs can't open HEIC, so you often need to convert iPhone photos to JPG to share them.
Convert with HEIC to JPG — free and in your browser.
Why Apple switched to HEIC
HEIC is Apple's use of the HEIF container, built on the HEVC video codec. Applying video compression techniques to a still image roughly halves the file compared with JPG at the same quality, which matters when a phone holds tens of thousands of photos. Since iOS 11, "High Efficiency" has been the default camera setting on iPhones.
Why it causes so much trouble
HEVC carries a complicated patent licensing situation, so support never spread the way JPG's did. Windows needs a paid codec extension from the Microsoft Store to show HEIC in Explorer or Photos. Many websites reject it on upload. Android support varies by manufacturer, and plenty of desktop software has never added it. The result works flawlessly inside Apple's ecosystem and awkwardly everywhere else.
What HEIC stores beyond the picture
A single HEIC file can hold more than one image — that is how Live Photos keep a still and a short motion sequence together, and how burst shots and edit histories are stored. Converting to JPG flattens all of it to one still frame. If the motion part of a Live Photo matters, export it as video before converting.
Turning iPhone photos into a document
Because PDF cannot embed HEIC, building a document from iPhone photos means converting to JPG first, which most tools do silently. Anyone assembling receipts or scanned pages from a phone hits this constantly. It is also worth switching the camera itself: Settings > Camera > Formats > Most Compatible saves as JPG permanently, at roughly double the storage per photo.