Adobe Sign is usually used to send documents for eSignature, giving you live tracking capabilities as well as a portal to manage and save all eSigned documents. You can also send automatic email reminders, include branding, send documents in bulk, create webforms, integrate with various solutions i.e. O365, Outlook etc.
Adobe Reader is a free service, and does not garner legally valid eSignatures. Acrobat PRO DC includes an individual Adobe Sign license with limited capabilities. For a feature comparison page with all the different packages and features of Adobe Sign, please go to: Compare Adobe Sign plans | Adobe Document Cloud