The global variables for a project are listed under the node Global
Variables 
under that project and they will include system
global variables as well as those defined for a particular project, see
Global variables overview
for more. For example there may be text that you want to reuse in many
actions, or email addresses used throughout a project. These can conveniently
be treated as variables so that any alteration is made only once and then
automatically updates all actions that use the variable throughout the
project.
To create a new global variable
select Global Variables under the required project 
then choose the New icon 
from the top of the
window or the toolbar. A new row with default entries is inserted under
the Name and Value headings, and you can simply type over these with your
new entries:
Name: Displays the name which will be used to refer to this variable throughout the project. System global variables are named using the convention of using capital letters and beginning and ending with underscores, and because of this they are listed together in alphabetical lists see Global variables summary for a list of all system global variables with details of their use. Project global variables which are what you define here, apply just to the current project and do not need to meet any naming conventions.
Value: System global variables do not have values set here, but those specific to a project do. During action execution the system automatically uses the variable's Value when it encounters the variable specified in the Name column. When you change a variable value, then every instance of the variable will take on the new value. For example if a global variable is defined with the value of the email address you can alter it here and have the system update all actions containing it - this is far more convenient than to manually update multiple actions - the change is automatically applied to all actions using that variable.
See step 1.Create Global variables of the chapter How to provide a web service for an illustrated example of creating a global variable.
Related topics include Global variables overview, Global variables summary