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i think this is not a unity question but…
http://en.wikipedia.org/wiki/Half-precision_floating-point_format
It might be Unity related, as you can use hald and float inside the shaders.
From the CG language ref:
The float type is as close as possible to the IEEE single precision (32-bit) floating point format. Profiles must support the float data type.
The half type is lower-precision IEEE-like floating point. Profiles must support the half type, but may choose to implement it with the same precision as the float type.
wow,clear answers ,thanks
halves (2byte floating point numbers) are famous in the gaming engines world used in shaders like marrrk said, and you can sometimes spot them in 3D game files like NFS Shift, they make the files smaller and are a direct feed to the shader inputs