The "Bell" icon will show you any new system notifications as well as "Mentions" sent to you from another user. Your notification preferences can be set by clicking your initials in the top right hand corner and selecting "My Settings".

After selecting the "My Settings" option from your user account button menu, click on Notifications. The feed column are your in app desktop notifications, the push column is your in app mobile notifications and the desktop column allows you to utilize the google chrome notifications on your desktop. See My Settings for details on how to customize your notifications.