A mechanical engineer designing machines deals with two things at the very basic level
2. Mechanism (a logic of motion or action)
Every machine has these building blocks from a nut and bolt to a car or a plane.
Engineers during analysis often need to break down each system into its smallest element which either constitutes a structure or a mechanism and then analyse it.
This might look as a very simplistic concept, but It is one which is most important. Large machinery and systems can be very intimidating for an engineer and can cause a lot of confusion when dealing with the details. If he/she can understand the tiniest fragments of the machine and its function as a structure or a mechanism, then the whole process of understanding and design becomes simple and comes naturally.
As an example : The frame of a car might look like a huge complex structure with many different types of brackets, mountings , members etc . But if you think that each bracket can be treated as a simple cantilever beam one at a time then the whole project of design can be broken down into manageable fragments which can be isolated from each other and worked on.
Often the mechanism element is combined with the structural element.
The connecting rod of an engine is a part of a mechanism as well as a structure.
Hence its design is driven by two approaches
1. Design for optimal motion and inertia
2. Design for structural strength
Often these requirements are opposed to each other and an experienced designer makes a well balanced trade off between them.
Civil engineers deal with structures for the most part but mechanical engineers deal with both mechanisms and structures.