Ethics are basically the measure of what is "right" and what is "wrong." In some circumstances ethics vary from one group of people to another, as in abortion and gay rights. In other circumstances they are almost universal, as in rape and charitable donations.
Ethical dilemmas -
Stem-cell research. Many agree that it is for the common good and leads to wonderful new medical advances, others feel that it shows a lack of respect for human life since the embryos used in the research are destroyed.
Genetic engineering. Some people believe that it could create new opportunities in agriculture, help solve hunger problems, and lead to cures for genetic disorders. Others believe that it is "playing God" or that mistakes could cause terrible consequences.
Animal testing. Studying the effects of different chemicals or procedures on animals helps us understand how those things will effect humans and solves many problems. It also does horrible things to the animals involved.