Safety testing
Many types of products are tested on animals to see if they have harmful effects. These products include not only human and veterinary drugs, but also cosmetics and toiletries, industrial chemicals, cleaning products, and pesticides and herbicides.
Obviously some of these products cause terrible suffering to animals. These are very cruel tests.
There are also practical problems. Animal tests are slow and expensive and they can't keep up with the thousands of new chemicals being produced. Animal tests are unreliable because of the differences between species. How a rat responds to a chemical is not necessarily the same as the way a human responds.
In the last few years many alternative tests have been developed to assess the safety of products. You can find detailed information about the problems with animal tests, and what the alternatives are, in these documents:
Tests for skin and eye irritancy
Tests for toxicity
Tests for chemicals that cause cancer or birth defects
These documents are quite detailed. If you want fairly short and simple descriptions, go to these documents first:
Drug testing