Alt is an abreviation for altered which is refering to the altered scale (the 7th mode of melodic minor). Always keep in mind, every chord has a scale from which it is built. The G altered scale looks like this: G-Ab-Bb-B-C#-D#-F and the intervals: 1-b9-#9-3-b5-#5-b7 (pay close attention to the 5ths and 9ths).
I imagine that the standard 7 chord eventually gave away to a 7b9 chord which has more tension. It is a pretty safe bet that the 7b9 chord was harmonized from the 5th mode of the harmonic minor scale (sometimes refered to as the phrygian dominant scale). Using G (same as C harmonic minor) as an example: G-Ab-B-C-D-Eb-F or 1-b9-3-4-5-b6-b7. See how this scale makes a G7b9 chord?
The altered chord is almost always a "V" chord going to the "I" chord, as in G7(#5,#9) - C maj7. But there are examples where this is not the case.