The MessageMedia Messages API provides a number of endpoints for building powerful two-way messaging applications.
The MessageMedia Signature Key API provides a number of endpoints for managing key used to sign each unique request to ensure security and the requests can't (easily) be spoofed. This is similar to using HMAC in your outbound messaging (rather than HTTP Basic).
The MessageMedia Webhooks allows you to subscribe to one or several events and when one of those events is triggered, an HTTP request is sent to the URL of your choice along with the message or payload. In simpler terms, it allows applications to "speak" to one another and get notified automatically when something new happens.
The MessageMedia Lookups API provides a number of endpoints for validating the phone numbers you're sending to by checking their validity, type and carrier records.