CRM stands for Customer Relationship Management.
An open-source CRM powered by Drupal, Civi can be customized to do nearly anything for your organization. Civi is powerful, but requires significant Drupal developer knowledge to get off the ground, and is not the easiest system for someone without tech skills to pick-up quickly.
Plenty of basic features bring both CRM and CMS together in one relationship. Although NationBuilder gives add-on access to the voter file, this file is lacking in comparison to the VAN file. NationBuilder has plenty of room for customization on the CMS side, but offers a standard menu of CRM options like: queries, supporter tagging, mass email messaging, etc.
Extremely straight-forward and easy to use, almost anyone willing to spend a little time learning can become a master of the ActionNetwork CRM. Integrates forms into any CMS using iframes and HTML embeds, which can be an up or a downside at times. Offers very basic options, with little room for custom solutions, or powerful engagement.