I’m rather new to blender, so as a simple test i created a low-polygon UVsphere. Then I added a subsurf to it, and set the level to 1 and the render level to 6. After applying it, I rendered it, but it was the same quality as in the editor. How come it isn’t rendering in higher quality?
You applied it, so you edited the base mesh to be the same as what you saw in the edit window.
The render settings were then thrown out, because you don’t have a subsurf mesh any more. you just have a normal mesh.
Hope that helps clear things up, welcome to the forums, and please post in the relevant forum next time. This is news and discussion, this is a support question, so next time please post this in the support forums.
What he said 
J.
BTW, using Subsurf on a sphere with Levels set to 6 is massive overkill. In a normal scene, your renders would take forever with no visible difference in the results than if the setting was say, 2 or 3. Some objects will not need Subsurf at all. You can also try Set Smooth and/or the EdgeSplit modifier.
With Subsurf, start at low Levels and increase them only when needed. How do you know when an increase is needed? Experiment. It won’t take long at all before you’re able to set the appropriate level more intuitively.
Indeed so, because increasing the subsurf by 1 level increases the face count 4 fold.
Cube:
level 0 means 6 faces
level 1 means 24 faces
level 2 means 96 faces
level 3 means 384 faces
level 4 means 1536 faces
level 5 means 6144 faces
level 6 means 24576 faces
It’s exponential, and why the level is limited to 6. It used to be 12, which would give a cube over 100 million faces.