Probably the best explanation on the GFF format, along with the most common versions and GTF, can be found at http://gmod.org/wiki/GFF3.
Somewhat related is the Browser Extensible Data (BED) format to specify annotations; this is mostly used interactively to specify tracks for genome viewers. You can find it at http://genome.ucsc.edu/FAQ/FAQformat.html#format1, although processing is quite trivial.
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