From the post:
When you turn on Battery Saver, your phone automatically turns off some of the features on your phone that use battery in the background. So when Battery Saver is on, your phone will no longer:
- Automatically receive email or calendar updates – You can still sync your email manually whenever you want. To learn more, see Sync my email.
- Update some Live Tiles – The technical explanation is that if a Tile uses something called push notification to get its updates, it'll stop receiving those updates in Battery Saver mode. But the upshot is that when Battery Saver is on, some of your Tiles won't be updated.
- Allow apps to run in the background – You'll only be able to use an app if it's open.
You can still make and receive phone calls and text messages, and browse the Internet.