The downside is that you're trusting your Google Voice and Twitter logins to third-party services, but if you wanna get push notifications about @mentions or DMs from Twitter or SMS messages from Google Voice without keeping a computer running at home with Growl whenever you're out, they're the way to go.

Any other services that use Prowl this way? [GVMax, Prey Fetcher, Thanks Zachary!]

Well, push describes a lot of things. Push is simply an action. Versus, say, pulling. Maybe that's horribly abstract, so try this: If information shows up on your phone or neural implant or messaging program without you (or your wares) asking for it—that's push. The info is pushed to you, versus you pulling it from the source. There are tons of ways push can be (and is) used.

Email's a pretty good starting point for grasping the difference between push and the other stuff. You probably know good ol' POP3—you log into your mail server and pull down new messages. Maybe it's on a frequent schedule, so it feels automatic, even instant, but you're still reaching out to the mail server every time to check and see if there's new mail to download.

IMAP is a little fancier than POP, where all of your folders and email are the same on all of your computers, phones and other gadgets, and any change you make on one shows up on the other, since it's all happening on a remote server somewhere. But with the standard setup, it's still the same deal—your mail program has to log in, see what's new, and pull it down. IMAP does have a pretty neat trick though, an optional feature called IMAP IDLE, that does push pretty well—it's what the Palm Pre uses for Gmail, for instance. Essentially, with IMAP IDLE, the mail server can tell whatever mail app that you've got new messages waiting, without you (or your app) hammering the refresh button over and over. When the app knows there's new messages, it connects and pulls them down, so it gives you just about the speed of push, without matching the precise mechanism.

While different systems do things differently (obvs), what true push services have in common is that they generally insert a middleman between you and the information source.

RIM's setup for the BlackBerry is probably the most sophisticated. When your BlackBerry registers with the carrier (which has to support BlackBerry), the details are handed to RIM's network operating center, so the NOC knows where to send your mail. The NOC watches your mail server, keeps tabs on the phone's location, and pushes email through to your phone whenever you get new stuff.

What makes it push is that your phone's not actually polling a server for new messages to pull—it only receives them when they hit your inbox, and are then pushed to your phone by RIM's servers. This means you save a lot of battery life that'd be wasted by making the phone constantly hit the servers for updates. The flipside is that when RIM's servers blow up, you don't get email, since it's all routed through their system—hence the other panic that grips dudes in suits once every few months lately.

The other biggie is Microsoft, who has Direct Push, part of Exchange's ActiveSync. It's architected a little bit differently, so it doesn't need the precise kind of data about where your phone is that RIM's NOCs do: The phone or whatever you've got sends an HTTPS with a long lifespan to the Exchange server—if new mail arrives before it dies, the Exchange tells your device there's new stuff, so it should start a sync. After it syncs, the device sends out another long HTTPS request, starting it all over again.

Apple's weak-sauce substitute for multitasking works pretty similarly: The developer has something its wants to send an iPhone, when its application isn't actually running, like an IM. It sends the notification to Apple's push servers, which send the notification to the phone through a "persistent IP connection" the phone maintains with the servers. This connection, which is only maintained when push notifications are turned on, is needed to locate the phone, but still doesn't draw as much power as constantly pinging the mail server.

Of course, those aren't the only push systems around, and it's only getting more and more important as stuff gets shifted to the cloud. We haven't mentioned Android and Google Chrome, but both utilize push (or will) in different ways. Suffice it to say, Google Sync will soon be a major player in this game. But basically, all kinds of different data can be pushed—calendars, contacts, browser data, hell, even IM is a kind of push—and they all work more or less the same broad way. Just don't ask us why there isn't push Gmail on the iPhone yet.

Still something you wanna know? Send questions about pushing, shoving and pancake massacres to tips@gizmodo.com, with "Giz Explains" in the subject line.

iTweetReply is a full-blown regular Twitter app for $2, though it's not the prettiest or most feature-packed—except for the push notifications for mentions and direct messages. Boxcar, on the other hand, works with the Twitter app you already use—when you get a push notification of a mention or DM, it'll open your favorite Twitter app. Interestingly, it costs more than iTweetReply, at $2.99.

Since DMs go to me as text messages, I don't need push enough rightthissecond to shell out another $3 on top of the money I've already shelled out Twitter apps. They'll get around to it. [iTweetReply (iTunes), Boxcar - Thanks Marsala]

The possibilities really are endless: You can be pinged with a push notification when a torrent finishes downloading, you get a new IM or email, you're mentioned on Twitter, or anything else Growl can notify you about.

You can customize which notifications are pushed to your phone and when they're sent (like only if you're not at your Mac), and Prowl stores up to 30 days of them. Way awesome, since it effectively makes iPhone push notifications infinitely extensible. Update: Ooo, support for Growl for Windows is coming soon too (thanks Samsita!). [iTunes, Prowl via Daring Fireball]

The app does what was touted this year at the iPhone 3.0 event: display notifications when the app is closed, show you how many outstanding messages you have and basically keep you "connected" to the AIM service even when you're not actively using the app. The free version is here and the pay version is here.

We personally would go with the free version until the pay version of the better IM apps (Beejive, for example) go live. Boy Genius says it's live now, but we're still seeing the old version in the store. [TechCrunch]

Android and especially the Pre have made background apps and the true multitasking they allow look a lot sexier, and the iPhone's one-app-at-a-time paradigm more restrictive, even with the innovative compromise of push notifications.

Push notifications, for the uninitiated, would allow apps like AIM to send you notifications (through Apple) of say, new IMs via an SMS-like prompt, even while the application isn't running. So you could kind of think of the app as running in the cloud, essentially. Not multitasking by any means, but for some an acceptable compromise on battery-draining background apps.

MacRumors says that if what they're hearing is true, and Apple allows apps to run in the background, it would happen with the iPhone 3.0 software. On the current iPhones, it would likely be restricted to one or two processes at a time, but with the beefier hardware of the next-gen iPhone, it'd be less restricted.

Having apps actually run in the background might actually be worth the longer wait. [MacRumors]