Action Center is a centralized location to view important messages about security and maintenance for your computer
It replaces the Windows Security Center from previous version of Windows
Action Center manages your Firewall settings, Windows Update, Anti-Virus and Anti-Spyware, Backups and Internet Security, it also monitors your computer maintenance and advises on any problems found and offers troubleshooters and tools that can help fix any problems
To access the Action Center right click on the flag icon near the clock 
and choose Open Action Center
It is divided in to four main sections:-
Security |
Shows you the status of your virus and firewall protection along with Windows update and internet settings |
Maintenance |
Allows you to check for solutions to any problems encountered as well as advising on installing updates and performing regular backups |
Troubleshooting |
Access troubleshooting packs which will guide you through and solve common issues you may encounter (see more details in Troubleshooting section) |
Recovery |
Open System Restore to allow you to roll-back your computer to an earlier date to undo recent system changes (leaves your documents, pictures, music etc unchanged) |
