I don't understand the difference between solidiers, navy seals, etc!??!!!?

There are all these different types of people. There are soldiers, navy seals, and way more. What is the job of all these different people? What are the other types besides navy's and soldiers? Who are the ones the ones that go into war? Who are the ones that march with drums and trumpets? What about cadets? Are they all different? Please help me understand!! Thanks!!