what does this mean exactly? I see it often when casting spells.

It means the spell was resisted, I'm pretty sure.

Only for all nothing spells, for damage spell the target takes half damage(provided its not resistant in that case it takes no damage).

Well, actually there's a little more range to resisting a spell than that. Depending on their roll, the Magnitude and Duration of the spell will be reduced: 0-60 is no save, 80-100 is full save, and 60-80 scales between no and full effect. So yea, about half the time you see "Save versus spell made" is means they took no effect, and half the time they took at least a little.