GreenArrow Studio is an application for sending email messages. It is centered around three main concepts: mailing lists, campaigns, and statistics.
A mailing list is a set of subscribers. You can import to a mailing list, export from it, edit subscribers, segment down to a particular sub-set of subscribers, and store those segments for later use.
Campaigns are a send to the mailing list. You can setup a campaign, edit the HTML and text content that will go out, send a preview email, choose the settings for how the email will be delivered, choose a particular sub-set of subscribers to send to, and launch the campaign for sending.
Statistics are how you view the results of an email campaign: how many addresses clicked, opened, unsubscribed, bounced, etc.