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Niko, it has to do with how the scope is mounted. Either Alt/Az or Equatorial can be goto and tracking mounts.
If a star is east of a line drawn from north to south (meridian) then an alt/az will track that star by moving west then "up" continually. Once the star moves past the meridian the mount will move west then "down". You will not notice this visually. But if you are imaging then you will get "field rotation". ie all the stars in the field will rotate around a central point. To do DSO imaging with this sort of scope you need a way to "polar align" it and this is usually achieved by using some form of equatorial wedge that tilts the mount head to the angle of your latitude.
An equatorial mount is set up to point directly at the south celestial pole (raised at the angle of your latitude) and when it tracks a star it does it only in one axis which will follow the star smoothly across the sky in an arc. You still need to guide to keep exactly on the star but you wont have field rotation (assuming you are polar aligned well).
Does that make sense?
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