Sampling

The quality of the renders can be adjusted by changing the anti-aliasing method. A different one can be selected for the 3D Viewport, Viewport rendering and for final rendering.

The setting for the 3D Viewport is a user preference to specify the anti-aliasing method that runs best on the used system. The setting for viewport rendering and final rendering is saved per scene.

Reference

Panel:Render ‣ Sampling Preferences ‣ Viewport
No Anti-Aliasing
With this option selected no anti-aliasing will be applied.
Single Pass Anti-Aliasing
Scene will be rendered with an anti-aliasing post processing pass.
Multisample

Scene will be rendered multiple times with a slight offset. The anti-aliasing will be gathered from the multiple renders. The number of samples are predefine so it uses the best distribution of the samples.

5, 8, 11, 16, 32

Vihje

Multisample anti-aliasing is well suited for rendering small details like hair.

Progressive Viewport Rendering

For the 3D Viewport, one sample is rendered at a time. When there are no changes to the scene or viewport the next sample will be rendered.