If you use Google's services for email, calendars, and contacts, you're probably aware that there are several different ways to access your data. You can use your favourite browser to access the Web interface. If you prefer dedicated Mac or iOS client app, you can use any that work with the industry-standard IMAP or POP protocols to access email, with CalDAV to access calendars, or with CardDAV to access contacts. On iOS devices, you've also had the option of using Google Sync--Google's implementation of the Microsoft Exchange ActiveSync protocol--for all those kinds of data.
Even though Google supports such open standards such as IMAP, CalDAV, and CardDAV for its mail, calendaring, and contacts services, you might have opted to use Exchange ActiveSync instead for two main reasons. First, it offered one-step setup: You enter your credentials once, and your email, calendars, and contacts were all set up at once, with no further fuss. Second, Exchange ActiveSync offered push delivery of email; if you're using IMAP to access Gmail on an iOS device, you've had to settle for fetching email either manually or on a schedule (every 15 minutes, say).
However, Google recently announced that--as part of its "winter cleaning"--it will be discontinuing Google Sync for consumers effective 30 January, 2013. So what might the loss of Google Sync mean to you? Google offers a helpful FAQ on the subject, but I'll run down the highlights.
First the good news: Most iOS users won't even notice the disappearance of Google Sync. For one thing, all email, calendar, and contact data will remain intact after January 30, and you'll still be able to use email, calendars, and contacts via the Web interface as you always have.
Furthermore, if you've already set up an iOS device to use Google Sync with Mail, Calendar, Contacts, or third-party apps, they will all continue to work without any modifications. And, if you have a paid Google Apps (for Business, Government or Education) account, nothing changes at all. The discontinuation of Google Sync applies only to those with free (consumer) accounts, and even then, only when setting up new devices to use the service on or after January 30.
Finally, even though Google Sync will be discontinued, you will still be able to access all of your data with your favorite iOS apps on any new device you set up; you'll just have to go through a slightly more complicated configuration process because you'll be using different protocols (more on this in a moment).
On to the bad news. If you have a free Google account, then starting on January 30, any iOS devices that aren't already using Exchange ActiveSync to access your Google data can't be added; anyone who creates a new free Google account won't be able to set up any iOS devices to use Exchange ActiveSync. Presumably, you'll be able to go through all the motions of adding the account and entering your credentials, but then the account will fail to validate and refuse to connect. Your options will be to sign up for a paid account (at $50 per user per year) or to configure your Google access differently.
If you want to set up an iOS device to access a Google account using the still-supported open standards (CalDAV, CardDAV, IMAP, and POP), you must currently use a two-step process. First go to Settings -> Mail, Contacts, Calendars, tap Gmail, and enter the requested information (your name, full email address, password, and account description). Tap Next. If you want to disable Mail, Calendars, or Notes for any reason, tap the corresponding On/Off switch. Then tap Save.
Next, you have to set up your device to sync your Google Contacts. Tap Add Account again, but this time tap Other, followed by Add CardDAV Account. In the Server field, enter google.com; otherwise, use the same credentials you entered a moment ago for your email, calendar, and notes. Tap Next followed by Save. (You can find further setup details on this Google support page.)
Remember that you need not change anything on any iOS device that's already set up to access Google services using Exchange ActiveSync, but if you choose to follow the above steps on such a device (for example, so that all your devices behave the same way), you should then either delete the Exchange ActiveSync account on that device or, at least, turn off all its services (Mail, Contacts, and Calendars) to avoid duplicated entries.