I worked mainly on cars and heavy trucks. See my answer here:
Douglas Lemmo, P.E.
Your question is quite broad, but I'll try. I would first direct to the Internet. Google Mechanical Engineer and you can spend a lot of time reading about them. I would suggest you look at job postings for MEs which will greatly help you to understand what MEs do.
There are basically four types of engineers: Mechanical, Civil, Chemical and Electrical. Mechanical Engineers work on things that MOVE: things such as machines, airplanes, ships, automobiles, trucks earth moving equipment. They design, build, install, maintain and operate them. Civil Engineers design, build and maintain things that DO NOT move: buildings, roads, bridges, dams, earth works. Chemical Engineers develop, manufacture and apply chemicals to manufacture items. Electrical Engineers design, build, maintain and operate things that have to do with electricity: Electrical distribution systems, computers, electrical motors.
In manufacturing today when building something, it would be very common to have several engineers of all of the above areas forming a team to build something like a power plant or a chemical manufacturing facility or a ship.
I think the great thing about mechanical engineering as a degree is that it allows you to diversify your knowledge in many areas that once you graduate can provide many career opportunities. You can work on research and development, testing, design, manufacturing, project management, and many more. Because ME is considered a very technical degree you can also land opportunities as an analyst or consultant because with the degree comes the implication that you are good a problem solving and data driven decision making.
You can work on doing tests out on the field, do simulations in a computer, or manage projects with an extended team. There are a multitude of options as a mechanical engineer, but also do know that you can always change jobs if you don't like your very first one it's very common in the industry to have people working on something that was not related to what they studied.