Starting with Leopard, the program /usr/libexec/path_helper was introduced to help manage the $PATH environment variable.

It is started by the /etc/profile file:

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if [ -x /usr/libexec/path_helper ]; then
    eval `/usr/libexec/path_helper -s`
fi

When path_helper is called, it searches various files, looking for new paths to add…

• it first reads from the file /etc/paths
• then reads files under the folder /etc/paths.d/
• and finally, oddly enough, the initial value of $PATH (prior to calling path_helper) is appended

Under the hood:

• path_helper only appends unique paths, so duplicates are avoided
• files from etc/paths.d are read in alphabetical order by filename
• the default value for $PATH (prior to calling path_helper) is /usr/bin:/bin:/usr/sbin:/sbin
• the default contents of file etc/paths is

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/usr/bin
/bin
/usr/sbin
/sbin
/usr/local/bin

How do I add more Paths ?

To add custom paths, you should create a plaintext file containing new paths (one per line) and place the file under the folder /etc/paths.d. Remember to end file with a newline!

Or from the terminal you can:

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sudo sh -c 'echo "/some/other/path" >> /etc/paths.d/another_paths_file'  

How do I make Homebrew happy ?

brew doctor commonly complains that /usr/bin is in your PATH before Homebrew's bin.
To fix this, move /usr/local/bin to the top of list in /etc/paths.

NOTE: The above changes are system-wide and will affect all users. If you only want to modify $PATH for a specific user, edit ~/.bash_profile, ~/.bash_login, or ~/.profile instead.