Instant messaging. If you don’t know what instant messaging is, there’s a teenager near you who does.

Instant messaging combines the privacy of email and the immediacy of the phone. You type messages in a chat window, and your friends type replies back to you in real time.

In this regard, Messages is a lot like the popular standalone messenger apps from AOL and Yahoo. But Messages lets you type back and forth with anyone from the AOL, Yahoo, Google Talk, and Facebook chat networks, all in a single program, which is a huge advantage. And Messages’ visual design is pure Apple.

Unified chat/text messages with phones (iMessages). This is huge. If you and your conversation partner both have iCloud accounts, then you can move freely from phone to tablet to Mac. Your conversation is auto-synced between gadgets. If you started texting someone on the train home, you can sit down at your Mac and open Messages—and pick up right from where you left off.

You can also fire off a text message to somebody’s phone while seated at your Mac—a mind-bending violation of the usual rules that limit text messages to the cellular network only.

The messages, which Apple calls iMessages, are a lot more flexible than regular text messages. They can be much longer than 160 characters. They can include photos, movies, or other kinds of files. They give you feedback that lets you know when a message has arrived on the recipient’s gadget.

And above all, they don’t count as text messages. They’re routed through the Internet rather than the cellphone voice network. As far as your cellphone company is concerned, you’re not texting at all, and therefore you don’t have to pay for a texting plan.

Free long distance. If your Mac has a microphone, and your buddy’s does, too, then the two of you can also chat out loud, by talking, using the Internet as a free long-distance phone. Wait, not just the two of you—the 10 of you, thanks to Messages’ party-line feature.

Free videoconferencing. If you and your buddies all have fast Internet connections and cameras like the ones built into most Mac models, then up to four participants can join in video chats, all onscreen at once, no matter where they happen to be in the world. This jaw-dropping visual stunt can bring distant collaborators face to face without plane tickets—and it costs about $99,900 less than professional videoconferencing gear.

File transfers. Got an album of high-quality photos or a giant presentation file that’s too big to send by email? Forget about using some online file-transfer service or networked server; you can drag that monster file directly to your buddy’s Mac, through Messages, for a direct machine-to-machine transfer. (It lands in the other Mac’s Downloads folder.)

Presentations. You can open up most kinds of documents, at nearly full size, to show your video buddy, right there in the Messages window, without actually having to transfer them. That’s Messages Theater.

Screen sharing. Next time some parents or neighbors are throwing themselves upon your mercy for tech support, remember this: Messages lets you see their screen, and even control it, from across the Internet. Or you can volunteer your own screen for sharing, so you can demonstrate things to them.