imessage and text message

A message is sent using WiFi to someone else that has the Messages app/application. A text message is sent using cellular data and can go to someone using Messages or someone whose device doesn't have the Messages app.

iMessage is Apple’s proprietary internet messaging service. It only works between people using Apple devices.

SMS/MMS is a cellular telephone texting service offered by all cellular telephone voice network service providers that predates, and has nothing to do with, the internet.

See - About iMessage and SMS/MMS - Apple Support