1. A good example of asexually-reproducing organisms are amoebas (small organisms made up of only 1 cell). In asexual reproduction the offspring is a genetic clone of the parent. Therefore, a population of asexually-reproducing organisms would have very little variation between individuals. Furthermore, the population would not change much from generation to generation because each new generation of offspring is genetically identical to the parent generation.
2. A good example of sexually-reproducing organisms are human beings. Sexual reproduction requires 2 parents (instead of just 1 like in asexual reproduction). The offspring of sexual reproduction are given a unique combination of each parents genes, which results in genetically different offspring. Therefore, a population of sexually-reproducing organisms will have a lot of variation between individuals. Furthermore, the population will change from generation to generation.
3. Asexually-reproducing organisms reproduce more efficiently because it only requires one parent, and therefore much less energy than sexual reproduction. In other words, it's much simpler and quicker for an amoeba to split up into 2 amoebas (asexual reproduction) than it is for 2 humans to reproduce sexually and nurture the child until it's ready to be born. The positive side of sexual reproduction, though, is how diverse the offspring can be. This gives populations of sexually-reproducing organisms a great advantage to survive because there is a higher chance of the some individuals in the population having beneficial adaptations.