Dry
This accounts for about 90% of cases and is caused by degeneration of the RPE layer that sits under the retina and nourishes it.  Waste products of the light sensitive photoreceptor cells build up under the RPE and can be observed as yellow deposits.  The overlying photoreceptors die, the retina thins and eventually disappears (usually in a fairly disecrete way in the centre of the macula).  Although slowly progressive, visual loss can be profound with dry AMD.

Wet
A chemical messenger called vascular endothelial growth factor, stimulates the development of new blood vessels from the choroid (the vascular layer beneath the RPE).
When these new blood vessels leak or bleed, macula can be damaged quite suddenly.  Fluid under the retina can lift up and stretch the macula, which results in distortion and blurring of vision.
Often patients with dry macular degeneration will developed sudden worsening of their symptoms or new distortion.  This is likely to imply the development of new blood vessels (a choroidal neovascular membrane or CNV) which needs immediate referral and treatment.
If untreated, the blood and fluid get replaced by a scar tissue and vision is irretrievably lost.