On the Account Permissions tab of your Google Account, you can see a list of third-party sites and applications. These are sites and applications to which you’ve granted permission to access your Google Account, and you can see on this list to what parts of your account they have access.
For example, you might have downloaded an app that helps you schedule workouts with friends. This application might have requested access to your Google Calendar and Contacts to suggest times and friends for you to meet up with.
There are several levels of access an application can have to your Google Account, but here are few examples:
"Full account access"
When you grant full account access, the application can see and modify nearly all information in your Google Account (but it can’t change your password, delete your account, or pay with Google Wallet on your behalf).
Certain Google applications may be listed under full account access. For example, you might see that the Google Maps application you downloaded for your iPhone, or the Picasa Web app on your desktop, has full account access.
You might also see sections for your Android or iPhone. If you ever lose your phone, you can revoke access to the Google services on that phone. If you later find your phone, you'll just need to sign in with your username and password before you can use Google services on it.
This "Full account access" privilege should only be granted to applications you fully trust, and which are installed on your personal computer, phone, or tablet.
If you've granted full account access to an app you don't trust or recognize, we recommend that you revoke this permission by clicking the Revoke access button.
"View your basic profile information"
These apps have access to basic data from your account, like your name, email, gender, or country. You might also see that the app can "Sign you in using your Google Account." That means that you can sign in to these apps with your Google username and password as long as you’re signed in to Google, saving you the hassle of remembering new passwords or creating a new account.
About read and write access
Permissions for some apps and sites might include read and write access, which means that they can post information about your activity on their app or site to Google products you use. For example, you have an app on your phone that lets you track how far you run. If this app has read and write access to Google+, it might post the number of miles you’ve run to your Google+ page.