APO = Apochromatic
It basically means one or more glass elements within the lens is of a higher quality than a non APO lens. Sigma terminogy...other Sigma lens options (off the top of my head) the 70-300 offer both APO and non APO versions...about £50 difference in price. Well worth it if you're after the sharpest lenses. Nikon call the same sort of thing 'ED' (extra low dispersion glass), Tamron 'LD' (low dispersion)...etc ....
Pick up a free Jessops catalogue at your local shop, as there's some lens reference guides in there which explain those letters which make appearances on many lenses now.
I can't offer anything additional with regard to the above lens, but wife Jacky has a 100-300mm f/5.6 Canon lens, despite being quite noisy focusing mechanism, it is quick, and is quite a nice sharp one, particularly at the 300 end.
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