SMS (Short Message Service) is a service for sending and receiving short text messages using phone numbers as addresses. Since the world's first SMS was sent in 1992, it has been a standard feature of mobile phones worldwide. The basic limit is 70 full-width characters (160 half-width) per message; longer text is split into several messages by concatenated sending and displayed to the recipient as a single message.
SMS's greatest strength is that it requires no app installation and can reach anyone whose phone number you know. This high deliverability has led to its widespread adoption for identity verification (two-factor authentication) one-time passwords, financial transaction notifications, delivery notifications, and appointment reminders.
Sending fees vary by carrier but are charged per domestic SMS, varying by character count. Receiving is free. Unlike messaging apps such as LINE, SMS is basically delivered over the same mobile network control path used for voice calls rather than over an internet (data) connection, so it works even when data service is unavailable. Because of this, SMS can still get through when the internet side is congested or down during a disaster. That said, SMS itself can be delayed when the network is congested, so the practical rule is not to rely on SMS alone for safety checks. The successor standard RCS has emerged, offering features SMS lacks such as image sending and group chat, all phone-number-based.
That same reach also makes SMS useful to fraudsters. Smishing impersonates financial institutions or delivery companies, using fake messages such as "You have a delivery" or "Your account has been restricted" to steer recipients to fraudulent websites. Banks and delivery companies do not normally ask you to enter a password or account details on a page linked from an SMS, and such a request is reason enough to suspect fraud. Rather than tapping a link in a message you were not expecting, check your account through the company's official app or a URL you bookmarked yourself. Caller ID Spoofing - How It Works and How to Spot It explains how the number shown for an incoming call can be falsified. If a message looks suspicious, you can also paste its text into the SMS Scam Checker.