Self explanatory really, limit by characters option i.e.
Why? Well limiting by characters allows for a stricter control on the height of an element, where as words is a bit more ambiguous in the size it takes up. Handy for grid layouts when you need each DIV to be a set height to stay uniform in their rows.
<cms:excerptHTML characters='30' ignore='img'><cms:show my_description /></cms:excerptHTML>
Why? Well limiting by characters allows for a stricter control on the height of an element, where as words is a bit more ambiguous in the size it takes up. Handy for grid layouts when you need each DIV to be a set height to stay uniform in their rows.