If you purchase movies, music, and TV shows from iTunes, it doesn't matter what device you purchased them on, in most cases they're available to you via iTunes in the Cloud, part of iCloud, from anywhere. Aside from your iPhone and iPad, that also includes iTunes on your Mac or PC.
If you own and iPhone or iPad you almost certain have an Apple ID. It's what you use to log into iCloud and iTunes, and it's what a lot of new iOS 8 features like Continuity use to make sure you're you, and to keep your documents and data private and secure. In other words, your Apple ID is important. If you don't use two-step verification enabled — and you really should! — if you forget your password, the only way to reset it is with your security questions. That's why it's a good idea to review your security questions, make sure they're easy to remember for you but not easy to guess for anyone else.
If you didn't originally set up your iPhone or iPad with iCloud, or if you want to use a different iCloud account than you do for iTunes purchases, you may need to sign in and set up iCloud outside of the initial setup process. In some cases, you may simply want to add an additional account to your device, you can add an iCloud account at any time via the Settings app.
While iCloud is meant to be used mainly on your iPhone, iPad, and Mac, times may arise when you'll need access to your mail or calendar from a public computer. Perhaps your iPhone is dead and you need to use a friend's computer, or you're on holiday and don't have mobile data but there is an internet cafe or terminal close by. Either way, you can quickly and easily access mail, contacts, calendars, and many of the other iCloud services from any web browser.
Any Mac running OS X Lion or later comes with iCloud support built right in. If you've already got an existing iCloud account or need to set one up, it's incredibly easy to get all your contacts, calendars, and your iCloud mail account linked up with your Mac.
If you're a Windows user, that doesn't mean you can't take advantage of the iCloud service. While Mac users may have it built right in, Apple also offers an iCloud Control Panel for Windows users that lets you manage some of your iCloud settings from your desktop, including Photo Stream, calendars, contacts, and more.
If you want to access your iCloud mail from a non-iOS, Windows, or Mac mail program, or with an alternate smartphone platform like BlackBerry, Android, Windows Phone, etc. you'll need to set it up manually using the IMAP and SMTP standards. While it may not be as pretty, it will be functional, and once it's set up you'll have access to all your iCloud mail, wherever you are.
Using iTunes Match on your iPhone or iPad and keep finding duplicate songs in your albums? Turns out you aren't alone. Sometime during the days of iOS 6, a lot of iTunes Match subscribers started reporting the same problem. Unfortunately Apple hasn't yet provided a permanent fix. However, there is a workaround that seems to solve the mysterious duplicate song problem. So how does it work?
Photo Stream is built right into the Photos app on iPhone and iPad. It's part of iCloud that seamlessly syncs your photos any time you're connected to a wifi network. Take a picture (or screen shot) on your iPhone or iPad and Photo Stream will store a copy of that picture on Apple's servers and push copies down to all your other iPhones, iPads, and Macs. Photo Stream will store up to 1000 photos, or 30 days of photos at at time. If you aren't yet familiar with Photo Stream, we can walk you through everything you need to get started.
Your Apple ID is what you use to make purchases in iTunes and to link your iPhone and iPad to your iCloud account. If you buy Apple products online, you also associate those purchases with your Apple ID. If you change email addresses or don't have access to the email currently tied to your Apple ID, you can easily change it.