Google uses single signon and if you are logged into your gmail account and open other google service, you will be automatically logged into that service. That way if you open gmail.com in your web browser you will be again logged into the same account.But if you have more than one gmail accounts and want to access those email accounts simultaneously, google gives you an option to enable it via google accounts. Below are the steps to enable muliple account sign in for your google accounts.
Go to google.com/accounts and login with your google id. Your google account settings page will open where you can set your profile settings and other google products that you are using.
Now under heading “Personal settings” you will find an option named “Multiple sign-in” which will be off by default. Click on the edit button next to it and make the settings as shown below.
After saving above setting when you will open you gmail account and click on your email id on the top right corner of the screen you will be able to see one more option “Sign in to another account”. If you click it you will be shown gmail login screen where you can login with your second account. Once logged in you will now be able to switch email ids as shown below. You can also open multiple Gmail tabs , one for each account.
Please keep in mind that this is a feature for advanced users, and there are a couple things to watch out for:
1) Not all Google services support multiple account sign-in yet. For the services that don’t support it (like Blogger and Picasa Web Albums), you’ll be defaulted to the first account you signed in with during that browser session. So if you click a link from Gmail to Blogger, for example, you’ll be logged into Blogger with the first account you signed in with, even if you clicked the link to Blogger from your second Gmail account.
2) Google is still working on making Gmail and Calendar work offline with multiple sign-in. If you rely on offline access, you probably don’t want to enable this feature quite yet.
3) Multiple account sign-in only works on desktop browsers for now, so if you use Gmail on your phone’s browser you won’t see this option yet.
Learn how to grant temporary access to your gmail account to someone else.