There are still people with iPhones who have non-unlimited texting plans, and for those folks it matters if they are sending text messages instead of iMessages. In addition, people who "text" from their iPad and Macs or have an iPod touch as their primary communicator don't have text messaging available to them, so it's iMessage or nothing. Understanding which is which can come in handy.
Firstly, in the Messages app, your outgoing message bubbles are either blue or green. That color coding is key to knowing what's what. Let's start with blue ones:
If they're blue, that means it is an iMessage going from one Apple device (iPhone, iPad, iPod, or Mac) to another. These are free communication, be it text or image, and don't count against your texting plan. You will only see blue bubbles on your Mac because your Mac running Mavericks (or lower) can't receive SMS messages. With a Mac running Yosemite and a device using iOS 8, this will change.
When you see the green ones, that means it is a "regular" (read: "old fashioned") SMS/text message, usually to an Android or feature phone user, and those do add up against your texting plan, pictures included. As noted above, this also explains why some people you chat with regularly on your iPhone or iPad don't show up in the Messages app on your Mac at all. Here's a "green bubble" message example:
If you look closely at the entry area before you type anything, it shows what it intends to send in light gray. It will say either "Text Message" or "iMessage" to help prompt you, but it can be hard to read.
An easy way to remember what's what is the green texts = money. If you're still on a non-unlimited texting plan, that might matter to you, so this is a quick way to see and monitor which texts are costing you and which ones aren't.
Also keep in mind it's common in group text threads to have one or two non-iPhone users, and every time you reply to that, even if the person who sent it is an iPhone user, you'll still have it count against you because it goes out as a text to all those people, iPhone users or not.