With new iPhones rumored to make an appearance, it's likely that Tuesday's Apple event will see a rehash of all the features coming to iOS devices new and old in iOS 7. In case you need a recap before the other shoe drops, here's a primer on exactly what we know about iOS 7 so far.

New interface, new gestures

If you've seen anything at all about iOS 7, you'll have noticed that it's very different in appearance than any previous version of Apple's mobile operating system. There are new fonts, icons and interfaces have a flatter look, the OS plays with layers, and there's not a scrap of rich Corinthian leather to be found.

Every stock iOS app gets a redesign, with some of the most significant coming to Safari, Calendar, Camera, Mail, and Messages. System features get major overhauls, too; even the lock screen has been vastly simplified, with edge-to-edge wallpaper and a design that foregoes widgets and buttons for straight-up text labels.

A few new gestures are present in iOS 7, too. You can swipe up from the bottom of the screen for Control Center, pull down on the home screen for Spotlight, and swipe upward on an app while in the multitasking interface to force-quit an app.

Under the hood

The interface isn't the only thing changing in iOS 7--there's also a bevy of new features awaiting users.

Control Center: This iOS 7 feature gives you access to many commonly-used settings with the flick of a finger. You can enable or disable Airplane Mode, Wi-Fi, Bluetooth, and Do Not Disturb; adjust your volume and brightness; pause or play music; enable AirDrop and AirPlay; and quickly access your camera's LED light (for use as a flashlight), timer, calculator, and camera.

AirDrop: Speaking of AirDrop, Apple's wireless file-sharing protocol is coming to the iPhone: When you want to share images and files with friends nearby, just make sure they have AirDrop enabled (for contacts or for everyone, depending on your level of friendship); their contact image will then pop up in the share sheet.

Multitasking: Multitasking has also been completely revamped. Not only can apps now take advantage of full background multitasking, but the multitasking bar of icons where you managed your currently-running apps has said sayonara; it's replaced with a new multitasking carousel. Double-press the Home button in iOS 7 and your current screen zooms out into a series of screens--each representing an app you were using--with the respective app icon below. You'll be able to scroll through them, tap on one to enter that app, or swipe upward on a screen to force-quit the application.

Notification Center: iOS 6's notifications clearinghouse gets an upgrade in iOS 7 with a new Today view that collects information about your current day, as well as a new Missed pane that provides a list of any notifications you haven't yet reviewed.

Find My iPhone: If you lose your iOS device running iOS 7, there's also some good news coming to you in the form of an update to Find My iPhone. Activation Lock is a new iOS 7 feature that forces would-be thieves to enter in your iCloud name and password if they wish to erase and reactivate a lost iOS device. And even after you've erased your device, your custom Find My iPhone lost message still displays on the screen.

Speech, speech

Not only has Siri's interface changed, but Apple's personal voice-activated assistant gets a little smarter and added a little more personality. U.S. English, French, and German users now have two voices to pick from--male or female--and those voices have dramatically improved speech processing.

Siri also now uses Bing search instead of Google; those results display in-line, though, and you can also search Wikipedia and Twitter. Tasks Siri already had some facility with have been improved, as well: Apple notes that the assistant can now return calls, play your voicemail, and control iTunes Radio, with more rumored to await you when you finally get your hands on iOS 7.

Snapshots and Sharing

The Camera app looks to have been both redesigned and beefed up in iOS 7: Not only has the gray interface of old gone the way of the dodo, but certain iOS devices will now have access to live camera filters, a square shooting mode, and faster shooting.

On the Photos app side, you've got a whole new way of displaying images. The app now sorts your pictures into Years, Collections, and Moments, with collections of images appearing as a mosaic of thumbnails. iCloud Shared Streams (née Shared Photo Streams) will also let multiple people contribute photos to the collection, and those people can add both images and video into the stream. You'll also be able to see what your friends have posted recently in Shared Streams' new Activity view.

Safari & iCloud Keychain

The Safari browser on your iOS device scores some serious changes in iOS 7, too. Besides its new icon, the biggest changes in Safari are its new unified smart search field, a minimized interface, new swipe gestures to go back and forward, a new tabs view, shared links, and quicker access to private browsing.

Also hidden inside Safari is iCloud's new iCloud Keychain feature, which allows you to store your saved passwords and credit cards, and also generate (and save) random passwords for new accounts.

iTunes Radio

Looking for a Pandora-like way to stream your tunes? iTunes Radio--built into the new Music app in iOS 7--lets you stream music from featured stations and from those you create. Like Pandora, you can tune your stations by giving iTunes Radio feedback on individual songs and on your stations themselves, and you can purchase any song currently playing for your library, if you don't already own it. Unfortunately for international users, the service will only be available in the United States at launch.

App Store improvements

So long, update badge: The App Store of iOS 7 will offer automatic updates for your applications on Wi-Fi or Cellular, so you never have to manually download an update again (unless you want to). The App Store also replaces the Genius button with the Apps Near Me feature, allowing you to see what's popular in your area, and adds a new Kids category (based on age) for parents to peruse.

In the car ...

iOS 7, paired with vehicles from certain auto manufacturers, will automatically pop up on your car's dashboard screen and offer a commute summary, calling support, music controls, Maps navigation, and voice support for sending and receiving messages. Apple says at least a dozen car vendors have signed on, but it may be a little bit before those models hit the road.

It won't run on the iPhone 3GS

RIP, iPhone 3GS. After several years of service, the last of Apple's non-Retina smartphones will be put out to pasture; they can't run iOS 7. Nor will the original iPad or fourth-generation iPod touch. There are still plenty of iOS devices that do support iOS 7, however, including the iPhone 4 or later, iPad 2 or later, and fifth-generation iPod touch or later.

While these older devices can run iOS 7, however, they may not be able to take advantage of all the iOS features that are available on a new iPhone or iPad. Specifically, here's a breakdown of what features require which devices.

iCloud: iPhone 3GS or later; first-generation iPad or later; third-generation iPod touch or later.Panorama: iPhone 4S or later; fifth-generation iPod touch or later.Square/Video formats: iPhone 4 or later; third-generation iPad or later; fifth-generation iPod touch or later.In-camera filters: iPhone 5 or later; fifth-generation iPod touch or later.Photos app filters: iPhone 4 or later; third-generation iPad or later; fifth-generation iPod touch or later.AirDrop (requires iCloud): iPhone 5 or later; fourth-generation iPad or later; fifth-generation iPod touch or later.FaceTime over cellular: iPhone 4S or later; third-generation iPad or later; fifth-generation iPod touch or later.Siri: iPhone 4S or later; third-generation iPad or later; fifth-generation iPod touch or later. Requires internet access.Maps flyover and turn-by-turn: iPhone 4S or later; iPad 2 or later; fifth-generation iPod touch or later.

Even more stuff

There's plenty of other little tidbits and features in iOS 7 that Apple only hinted at WWDC and on its iOS 7 preview page, including new Mail features, a redesigned Compass app, a revised Weather app, and more. We've collected some of those already; for the rest, we'll have to wait and see what else Apple has in store for us come September 10.

And don't rule out any potential new iPhone-specific features showcased at the event. Apple loves to releases new operating system features alongside its newest models, like HDR mode, Panorama shooting, and Siri. The best new iOS 7 feature may not even be here yet.