Text Messaging, also called SMS (Short Message Service) allows short text messages to be received and displayed on the phone. 2-Way Text Messaging, also called MO-SMS (Mobile-Originated Short Message Service,) allows messages to be sent from the phone as well.
Messages can typically be sent to a phone via a special email address, through the carrier's web site, or with special messaging software and a modem.
Messages can be sent from one phone to another by addressing the message to the recipient's phone number.
Most phones and carriers also allow messages to be sent from a phone directly to an e-mail address.