Natural selection can be seen to favor certain genotypes/phenotypes, so the change in a population can be predicted. Genetic drift instead involves small populations and changes in alleleic frequencies driven by random chance, which cannot be predicted.
Work Step by Step
To see this, think of the following: if a predator eats yellow snails but leaves purple ones alone, the yellow phenotype/alleles leading to it will decline in frequency due to natural selection. In contrast, if there is a population of three yellow snails and four purple ones and I happen to step on all three yellow ones at once, killing all, allelic frequencies change due to random chance.