EXIF

EXIF tags are TIFF-like tags added to JPEG files, commonly used in digital photography for storing date, time, resolution, camera model, exposure settings and much, much more.

Practically every modern digital camera maufactured today supports EXIF tagging, and some smartphones do it as well (Nokia Series 60 phones do it in a remarkably obtuse fashion, but at least it's there in a way).

Tags of interest: ImageDescription (baseline TIFF), UserComment

• Wikipedia (recently updated)
• Bill's Notes on JPEG metadata
• EXIF 2.1
• Reference (PDF)

• jhead is one of my favourites
• exiv2 seems to do all jhead does, and more (including IPTC tagging)
• jpeg impec, an AppleScript bundle that uses jhead and jpegtran
• The PHP JPEG Metadata Toolkit - pretty much everything you might need.

JavaScript

• Reading EXIF data with JavaScript

Python

• jpeg.py - reads and writes metadata, restricted but simple to use
• EXIF.py (read-only)
• pyexif - pretty complete parsing (read-only)
• IPTCInfo (read-write)

Perl

• ExifTool is a Perl module and command-line tool that reads and writes EXIF tags, including GPS ones (table)
• an EXIF module written in Perl

PHP

• Exifer, a PHP library
• phpExifRW, the only PHP4 library I know of that actually writes EXIF tags.
• PEL, a PHP5 library that can also write EXIF tags

C

• libexif