You can send SMS/Text messages as well as emails through your system. As long as that Contact has a phone and/or email address, you will have the options to communicate with them.
TEXTING:
To send a text, simply choose "SMS" (instead of email), type your message and send. Note that the text will come from the phone/text number associated with your user account. This number is different from your cell phone which means that, when the Contact replies, those replies will show up in your Conversation, not on your cell phone.
EMAILING:
To send an email, choose "Email", then type your subject, message, and send.
Note that when you send an email, the message will come from your true email address. This means that if the Contact responds, their email response will show up both in Conversations as well as come directly to whatever your normal email inbox is. However, if you send an email from your normal email outbox, that message (and any subsequent replies from the contact will NOT show up in Conversations). So remember that if you want all emails to show up in Conversations, you need to be sending emails from Conversations, not your normal email system.