To answer this question, let’s first look at a typical text message. When someone sends a text message, they type a message on their phone and hit the send button. At that point, the cellular data service provider (Verizon, AT&T, Sprint, or whoever) transfers that message through their network to the recipient.
Depending on your cell phone plan, you could be charged for the text message (if you don’t have an unlimited texting plan or if you exceeded your texting limit for the month). While sending a few text message may not seem significant, a teenager’s appetite for texting can cause charges to add up quickly.
When you send a text message to someone that has an Apple ID, the Messages app automatically recognizes that person’s Apple ID and routes the message through Apple’s servers instead of using the cell phone carrier. This is done automatically and avoids any texting charges from the cell phone carrier. Sending iMessages will still use cellular data when the iPhone is not connected to a Wi-Fi network. Users that approach the monthly limit of their cellular data plan may opt to turn off iMessages. See this article for more information on disabling iMessage.
Since iMessages can be sent using a Wi-Fi network, they can be sent from an iPad, iPod touch, or Mac computer that has no cellular data connection.
The Messages app provides some visual indication as to whether you will be sending an iMessage or a regular text message. In the figures below, the one on the top shows the Messages app when you are sending an iMessage. The figure on the bottom shows the Messages app when sending a regular text message.
Notice the iMessage displays sent messages in a blue background and the word “iMessage” is shown in the text entry field. When sending a regular text message, the text entry field displays “Text Message” and sent messages are displayed with a green background.
I have updated this article to provide some additional information along with updated screenshots.
Other articles related to iMessages
If you have multiple iOS devices in your family and iMessages are being delivered to all your devices, please see my article about configuring multiple devices so iMessages can be delivered separately for each person.
If you question why text messages are delivered to your iPhone but not your iPad, please see my article describing text message and iMessage delivery.
Occasionally, you may notice iMessages are being sent as regular SMS text messages. This article describes why iMessages are sent as SMS text messages and how to avoid it.