Explain self-pollination and cross-pollination. Which of them is more useful?

Self-pollination or autogamy is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or from one flower to another on the same plant.

Cross-pollination or allogamy is the transfer of pollen grains from the anther of a flower to the stigma of the flower, borne on a separate plant of the same species.

Cross pollination is more useful because it results in combination or mixing up of characters thus, improving the quality or vigour of the species. Continued self-pollination results in loss of vigour and finally leads to sterility.