How do I integrate my Petition, Email Protest, Donation,... with any other site
There are a lot of ways to integrate with Campaingion. We won't cover all of them here, but these are the most important for supporter management and your forms:
If you create a form on any site (doesn't matter if it's a petition or something else) you can embed this form on any other website via your very own widget. Create a widget by clicking on the "Widget" tab at your petition. Copy the provided code and paste it wherever you want to have your form displayed. This doesn't work well with very long and complicated forms, so for a multistep form use different ways to get your supporters to submit it.
Newsletter providers (Mailchimp, Cleverreach, etc)
If you use the toolbar to go to Manage > Advanced Settings > Site settings, you will find the section "Web services" and "Newsletter settings". Select a machine name for your connection. This needs to be unique and can only contain lower case letters, numbers and underscores. In the next field, paste your API-key from your newsletter provider. You can set up several keys if you like by clicking on "Add another key". Save your configuration with the button on the right side. All your lists and subscribers will get updated regularly, if you want to update them right now click the button at the bottom labelled "Update Lists and Subscribers from all external sources now".
List of fields that are available
An actual list of synchronised fields for some newsletter providers is available here