It's not really any mystery what the navy does: Patrolling waters, performing rescues and, very, very occasionally, getting involved in sea battles. However, there are many jobs with the navy that don't fall into the obvious category of 'sailor'. From logistics to cooking, the navy, like the other armed forces, provides diverse career paths for people with all kinds of skills and qualifications, even if you're not sure which end the stern is (it's the pointy one, right?).
If you do become a sailor, there's pretty intense training involved, and you may be posted a considerable distance away from your family. There's also the risk of serious injury or even death, as well as long-term mental disorders. For this reason, it's not a decision to be taken lightly. As an active member of the navy, you may be station on ships, in submarines, at a naval base or even working as a pilot (yes, the navy flies, too).
As stated above, there are probably hundreds of different careers that you might have working for the navy, all requiring a distinct set of responsibilities and skills. What they have in common is the armed forces focus on rigid discipline and hierarchies. You will be expected to receive and follow orders and there won't be a lot of room for making suggestions or asking questions. So, if you're a bit of a free spirit and don't like to be told what to do, the navy is probably not for you.