In the field of science performing experiments is the foundation of building knowledge. Likewise, in the field of medicine, testing medications and procedures is extremely important. However, because these tests are not proven, it would be very dangerous to perform them on human beings. For this reason researchers use model organisms.
So we know that model organisms are used to perform tests and experiments before they can be done on humans, but how is this possible? Well, because the role of proteins and the interaction of genes and proteins is the same in all living things, we can use organisms that are similar to human beings to understand how things occur in humans. For instance, by testing dugs on mice, we can get a good idea of the effects on human beings. Cancer researchers have an additional model on their side though: cancer itself. Because cancer grows so rapidly and is so resistant to death, scientists have created cell lines that are living cancer cells in a petri dish.
Cancer cell lines are somewhat unique in the ease that they can be grown in a dish, and a significant amount of cancer research is performed on cells living in glass dishes. For this reason, much of this testing is referred to as In Vitro (Latin for "In Glass") testing. The cells used in research typically came from a cancer patient, and each of the cell lines that are used is different from the others, either coming from a different part of the body (lung, breast, ovaries, etc.) or expressing different mutations that might impact treatment. Even better for science, because Mitosis creates duplicates of cell, all researchers across the world are using cells that are the same, and thus results can be shared and replicated easily.
While cell lines have their place in research, the environment in a petri dish is a lot different than being in a living organism. For this reason In Vivo (Latin for, "In Living") tests of different kinds are still done. Testing in medicine is often done on mice but other fields of science use a variety of organisms, including yeast, flies, and monkeys. Tests on living things are important because a great deal of the biology taking place in an organism is still not understood, and surprising reactions can occur in either in vitro, or in vivo testing but not the other. Accounting for this is important in Drug Development to ensure the safety of humans involved.