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Genetic drift is a change in gene frequency within a population due to chance instead of natural selection. If the population is small, then there can be considerable genetic drift, which can change the gene frequency quite drastically. Think of the genetic variation of a population as a bowl of jelly beans with all sorts of colors, representing healthy genetic variation.
Founder effect and population bottleneck are not necessarily examples of genetic drift. The common thing in all three is a resulting reduction in genetic diversity. Genetic drift is a change in allele frequencies due to process of random sampling to form a finite offspring population from parent population.