Mechanical engineering is exactly what it sounds like - a discipline which uses materials according to the laws of physics to make production, locomotion and transportation easier. Building and maintaining machines - oil rigs, power plants, airplanes, ... - all this falls under the umbrella of "mechanical engineering".
In the beginning, a mechanical engineer usually has to do a fair amount of physical work - like a mechanic, as it were. However, things grow easier with time, and after a certain degree of seniority, one can stick to supervision, administration, drawings or other tasks that do not involve physical labor.
There are six sub-disciplines to mechanical engineering, all of which generate medium-to-high incomes even at the starting level.
1. Mechanics. Mechanics deals with the effect of forces on matter, and of course the forces themselves. These are used to determine how bodies will react under certain forces while in rest or in motion, how fluids respond to forces, and how bodies respond to stress and strain. Mechanics is most useful during design and analysis - it is used to create a design that utilizes forces most efficiently, and to nullify any flaws in the design.
2. Kinematics. Kinematics, quite obviously, deals with the motion of objects and bodies. However, the forces causing the motion are ignored. The movement of a crane or an engine piston are cases where kinematics would be applied. The science is used to figure out the range of motion of a given implement, or to design an implement with a predetermined range of motion.
3. Thermodynamics. Thermodynamics is the study of energy - enthalpy, entropy and transformation from one form to another. This is involved in all operations that involve energy changes - fuel utilization, heating and cooling, chemical changes etc. Air conditioners, engines and insulation systems are a few cases of thermodynamic implements.
4. Mechatronics. This sub-discipline covers more than just mechanical engineering. In fact, this is one of those hybrid areas where mechanical engineers just love to go. Involving software engineering and electrical as well as mechanical, mechatronics can create systems that perform motor functions through electrical stimuli with the help of software. For example, the hard drive, or - more visibly - the optical drives on a computer. Robotics is another sub-branch of mechatronics, and this is used for the creation of robots. Many manufacturing industries employ robots for tasks that are repetitive, repugnant or dangerous to human beings.
5. Structural analysis. Structural analysis is the discipline of finding out the causes of failure in a particular machine or structure. Usually, the cause of failure - breakage, excessive heat, friction etc - can be pinned down to flaws in the implement's design or chemical composition.
6. Drafting. An essential skill for all engineers, drafting is the process of creating an accurate drawing from which a product can be assembled. The drawing must be perfectly in scale, and include information about the materials to be used, joins to be placed etc.
Moving from one to another sub-division of a mechanical engineering job is fairly easy. Through the mechatronics route, a mechanical engineer can even move on to working in the hardware or software sectors, which pay much better than equally gruelling jobs in, say mechanics or kinetics. The skills of an engineer are always in high demand, so students can take up this line without doubt or fear.