Question: I want to customize the look and feel of my `xterm` and have read that the way to do it is creating a `.Xresources` file and saving there the desired configuration. I've also learned that this file can control so much more than just the look of `xterm`, for example the screensaver and the mouse, among other things. My question would then be: if I create a `.Xresources` file and just write the settings for `xterm`, what happens to my screensaver, mouse and other stuff controled by `.Xresources`?

Answer: 
Only the values you explicitly define in your `$HOME/.Xresources` will override the system defaults.

So you could specify some settings [as sugested by the Arch Wiki](https://wiki.archlinux.org/index.php/X_resources#Xterm_resources), and they would apply to any instances of xterm opened by your user, but all other settings would remain unchanged.
In a [`.Xresources` file](http://www.x.org/releases/X11R7.7/doc/man/man7/X.7.xhtml#heading14), settings consist of a resource name and a value. A resource name has several components separated by a dot. The first component is the name of the application, subsequent components are widgets in the application, and the last component is a setting of the widget.

A component name that starts with a capital letter is a class. For an application, that's the application name (e.g. `XTerm` for Xterm, unless overridden with the `-class` command line option). For a widget or individual setting, that's the type of widget or setting.

A component name that starts with a lowercase letter is an instance. For an application, that's the name of the executable (e.g. `xterm`, unless executed under a different name such as `uxterm` or overridden with `-name`).

You can also use the wildcard `?` as a component name, and `*` to mean any sequence of subcomponents (so `*` means `.` or `.?.` or `.?.?.` or …).

Any resource specification that doesn't start with a wildcard will only apply to the specified application. (In practice, you can't really make useful non-application-specific resource specifications, as there's no unification in widget names.)

Any resource specification in `.Xresources` (or loaded otherwise) overrides the application's built-in defaults for that resource. (If wildcards are involved, complex precedence rules apply.) Other resources are not affected. Thus you don't need to copy any of the default settings, just write the ones you want to overwrite.

There's one exception of sorts: if you define key bindings, these are all specifies in a single resource (in the `Translations` class), so your set of bindings replaces the default set. To augment the default set instead, start the translation table with `#override`. Appendix B of the [Xt Intrinsics documentation](ftp://ftp.x.org/pub/current/doc/libXt/intrinsics.pdf) has all the details of the translation table syntax, but you don't need to read that, the examples in the Xterm man page are enough to grasp the useful bits.