Answer
Sampling risk is the risk that the auditor's conclusion from a sample differs from the conclusion had the whole population been tested (because the sample isn't fully representative). Non-sampling risk is the risk of a wrong conclusion for reasons unrelated to sample size—like using an inappropriate procedure or misinterpreting evidence. Larger samples reduce sampling risk; better performance reduces non-sampling risk.
Real-world example A sample happening to miss the errors gives false comfort (sampling risk); misreading a document gives a wrong result (non-sampling risk).