JPEG
JPEG is the most-used photo format for web and sharing: small, universal and openable everywhere. It reaches that small file size with lossy compression, which throws away a little image data each time. Ideal for the final result, but not for editing in.
Export your photo as JPEG for web, social and email, with sRGB as the color space and a quality around 80. Never edit in JPEG for long and don't resave it repeatedly, because each save compresses further and stacks up quality loss. Do your editing in RAW or TIFF and only export to JPEG at the end. A JPEG has only 8 bits per channel, so the leeway for heavy adjustments afterward is limited.
Read also the in-depth explanation: JPEG explained.
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