ActionID is your login credential that consists of your email address and a password. It can be linked to one or more user accounts, which provide you access to databases.
You might have more than one user account if you volunteer for multiple campaigns or consult for several campaigns and organizations. If you have more than one user account, your ActionID can be linked to all of them. With ActionID, you only need to remember one password, which you can reset yourself; and you don't need to remember usernames, just your email address.
Additionally, different user accounts may be on different sites - VoteBuilder, SmartVAN, etc. If some of your user accounts are on the same site, you can easily switch from one user account to another within that site. However, if your user accounts are on different sites, you will still need to go to the relevant site to be able to log into each user account. To confirm where you should log in for each organization, be sure to bookmark the web address provided in your login email.
ActionID can also be used to log into MiniVAN or Open Virtual Phone Bank so you can canvass or make calls without a "full" user account. This kind of access is still provided by the campaign or organization you're volunteering for in the form of a canvass list number or a phone bank code.
See also: How to Set Up Your New VAN Account