I want to have fish and plants. I also want to have clear water. If these are your goals, some sort of filtration is necessary. There are 2 types of filtration, mechanical and biological. These are used to keep the junk from clogging your pump and are usually at the inlet of your pump.
Mechanical filtration can also be something as simple as a settling pond.
Biological filtration works on a different level. Fish waste is composed of ammonia and nitrites, both of which will kill fish. Bacteria exists in the water that will convert these nasties into nitrates, fertilizer. Unfortunately, our ponds are not build to contain enough of these critters to do the complete job. A biological filter is designed to make a home for enough of them to grow and multiply and do their job.