Warmer fermentation temperatures usually translates to higher levels of yeast activity. For many popular ale yeasts, running 5°F warmer than "ideal" will significantly increase the rate of fermentation. As well, depending on the yeast strain, running as little as 5°F outside the ideal temperature range can significantly affect flavor characters generated by the yeast...
Every yeast that you purchase will have an optimum range of fermentation temperature. Do not be afraid to read manufacturer labels or visit the websites of the manufacturers, from whom you purchase your yeast, to research how to utilize their product to maximum effect.
depends on the yeast strain. try to stay within the parameters of that range.
as someone else already stated, the higher end of that range will lead to more vigorous activity. higher temps also cause the yeast to give off more esters. in some strains the higher end is also known to enhance hop attitude. be careful though because higher temps beyond the range will give you phenol flavors which are not that tasty.
i've found that the lower end of range, the cleaner the beer. lower beneath the range will slow or could possibly stop fermentation.