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Hi Benno
Short is f/5. Sometimes it's also called a "fast" focal ratio or a "fast" scope.
Long is f/10 and above. Sometimes it's also called a "slow" focal ratio or a "slow" scope.
I wouldn't necessarily say fast is better for deep-sky and longer is better for planets/moon. It just depends what you want to do. I love fast scopes for the wider field of view and more clear contrasty images they provide.
Fast scopes have more problems with coma and is more demanding on collimation and eyepiece design. Slow scopes just don't let as much light in, have a narrower field of view and more magnification for a given eyepiece.
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