In selective breeding farmers choose parents with the characteristics they want, breed them, select the best offspring and repeat this over many generations. It has produced crops with a higher yield and cattle that give more milk. It is cheap and needs no special equipment, but it is slow. It also reduces genetic variation, so inbreeding can occur, inherited disorders become more common (as in some pedigree dogs), and a population with little variation is at risk because one new disease could affect every individual.
Genetic engineering is faster than selective breeding because a single gene is transferred, and the gene can come from any species. In agriculture it can give crops that resist insects or contain extra vitamins. In medicine, bacteria make insulin for people with diabetes. The risks are that the effects on human health may not be fully known, the new gene could spread to wild plants, other organisms could be harmed, and the technology is expensive, so seed can cost farmers more.
There are also ethical issues. Some people object to moving genes between species or to modifying animals, and they worry about animal welfare and interfering with nature. Others argue that GM crops can reduce hunger and that GM medicines save lives. To evaluate, weigh the benefits against the risks, think about who is affected, and then reach a conclusion that you can justify.