SMS stands for Short Message Service, which is a text messaging service that uses standard communication protocols to let mobile devices exchange short messages. It allows users to send and receive messages up to 160 characters, including spaces, and does not require Wi-Fi or a data plan. SMS messages are sent over a cellular network and appear in green text bubbles on an iPhone. To use SMS on an iPhone, a text messaging plan is required.

On the other hand, iMessage is an instant messaging service developed by Apple in 2011. It is located in the default Messages app on an iPhone and is capable of sending both iMessage and SMS messages. iMessages are texts, photos, or videos that you send to another iPhone, iPad, iPod touch, or Mac over Wi-Fi or cellular-data networks. These messages are always encrypted and appear in blue text bubbles. iMessage requires an internet connection, either through Wi-Fi or a data plan, and is automatically encrypted.

The main differences between SMS and iMessage are:

• Availability: SMS is available on virtually every phone and tablet in the world, while iMessage is available only on Apple devices.
• Security: iMessage texts contain end-to-end encryption that regular SMS texts dont, which protects communications from getting intercepted by hackers or other unwanted third parties while going through networks.
• Features: iMessage offers a lot of cool features that SMS messaging doesn’t, such as sharing location, sending walkie-talkie style voice messages, getting confirmation that a message has been delivered, getting read receipts, and seeing whether someone is replying to a message in real-time.

In summary, SMS is a text messaging service that uses standard communication protocols to let mobile devices exchange short messages, while iMessage is an instant messaging service developed by Apple that allows users to send and receive messages, photos, videos, and other media over Wi-Fi or other cellular data networks.