Text messages sent via SMS are limited to 160 characters. This is because in 1985 wireless bandwidth was in extremely short supply, and characters had to be limited to a smaller number than ideal. One of the people on the team building the SMS standard found that commonly sent messages were well under 160.
Apple’s iMessage has changed the times, and changed with the times. It allows longer messages with more freeform text. It allows all fonts and characters, and a diverse emoji collection. This has changed how we speak to each other, and what we can say. But the biggest way it has changed communication between people day-to-day is in how it utilizes the data network. Not only are messages sent faster most of the time, but in addition media can be sent. Not just the low-resolution photos on cheap phones, but real, high definition several-megabyte images and tens of megabytes in video (iMessage videos cap at around 3.5 minutes).
Also, moving over the data network, these messages are much less susceptible to third-party eavesdropping on communication. Text messages are essentially in the open and accessible by any telephone company. iMessages, on the other hand, are stored only on devices and Apple’s own servers, which Tim Cook has publicly stated nobody else has access to (not that taking this with a grain of salt isn’t advised).
iMessage doesn’t even require a phone number, merely an Apple ID. One can iMessage friends to and from any Apple ID. However, one can opt to use the phone number as the identifier, and it’s bound to the user’s Apple ID. I personally use iMessage with solely my phone number.
iMessage also allows “Read receipts”, which can be sent to the sender on the other end, enabling them to know the message has been read by their recipient. This is a feature many users value, although it would be wonderful if Apple made it possible to select who does and doesn’t receive these read receipts. Currently the only way to do it is by installing unauthorized software via jailbreaking.
iMessage is available on all iOS devices. A couple of other fantastic features are the third-party keyboard options available to users and Siri dictation, both wonderful aspects of the iOS 8 interface.
One other company had an iMessage-like service, using data instead of the cellular network. Blackberry Messenger is still used by people today, but not much. It lacks iMessage’s popularity and robustness. My apologies to any die-hard BBM users I offend!