Ales ferment between 60 and 75 degrees while lagers should be nearer 50 degrees. Ales can be ready to drink in as little as 4 weeks but your lager should take longer as the yeast works slower at the lower temperature.
The yeast. Lager yeast are a different species from ale yeast. Also, lager yeast can handle cooler temps, tend to produce "cleaner" flavored beers, and can metabolize a wider range of sugars than ale yeast.