Directory structure on *nix machines

One of the most confusing things when doing basic server maintenance or troubleshooting on a remote machine is where everything is located. A basic understanding of the Filesystem Hierarchy Standard is helpful - though there are, of course, exceptions all over the place, and the standard isn't always consistent! The main directories you'll probably be interested in are:

/bin

Main system binaries that come bundled with the operating system are stored here. Generally speaking you want to keep out of this main bin directory. Everything in the /bin directory is available to all users.
/etc

This is probably the most confusing directory name - just as you may have assumed, this originally did indeed mean the 'etcetera' directory, i.e. everything that wasn't acounted for elsewhere. Binaries are not allowed to be stored in /etc and generally it's used to store configuration files for various programs. This directory is where you will find the configuration files for webservers like Apache (/etc/apache2) and nginx (/etc/nginx).
/home

User home directories are found here. MacOS also has a home directory, however that is generally empty, and instead MacOS uses a /Users directory.
/lib

This is for libraries associated with the binaries in /bin. These are things like templates and modules associated with the binary application files. In MacOS this is called Library.
/sbin

This is another directory for system binaries, but they are only available to the root user or other users in superuser mode.
/srv

Officially this is for "Site-specific data served by this system, such as data and scripts for web servers, data offered by FTP servers, and repositories for version control systems."

However - see notes under var, becuase website files usually are NOT put in /srv by default.
/tmp

Temporary files. Usually files stored here will be deleted on reboot. See also /var/tmp
/usr

Secondary hierarchy for read-only user data; contains the majority of (multi-)user utilities and applications. Mostly this has the same subdirectory structure as the root directory i.e. usr/bin, usr/local/lib etc
/usr/local

Tertiary hierarchy for local data. Mostly this has the same structure as the root directory as subdirectories i.e. usr/local/bin, usr/local/lib and so on. When writing your own executable scripts you generally should save the binary at /usr/local/bin because the other binary directories can be overwritten by the system when you do an operating system upgrade.
/var

For 'variable' files. This is for files that are expected to change a lot, such as logs, print queues and databases.

One important exception to this is that in many Linux distributions, including the very popular Ubuntu Linux, any website files go into /var/www/html by default. This doesn't really comply with the official standards, because they're supposed to go into /srv, but has become something of a de facto standard
/var/tmp

Temporary files are stored here and will be retained on reboot.

Finding the Desktop from Bash

MacOS

/Users/YOURUSERNAME/Desktop

~/Desktop

Windows (Ubuntu):

/mnt/c/Users/YOURUSERNAME/Desktop

Windows (Cygwin):

/cygdrive/c/Users/YOURUSERNAME/Desktop

Linux

/home/YOURUSERNAME/Desktop

~/Desktop