User profile synchronization service in SharePoint 2010 enables synchronization of Users and Groups data from directory sources such as active directory and LDAP.
1. Central Admin > System Settings >
2. Select Manage Services on Server
3. Scroll down and select User Profile Synchronization Service. If it says Stopped, click Start
4. When you click on Start, SharePoint 2010 will ask you to enter user user name and password. To import profile the account farm account HAS TO BE A LOCAL ADMINISTRATOR
Enter your password and click OK
5. Now your status should say Starting
6. After couple of minutes if you refresh, it would stay started
6. Do an iisreset . Go to Start and run and type in iisreset
7. lets go to Central Administration and navigate to Manage Service Applications
8. In Manger Service Applications scroll down and select User Profile Service Applicaitons
9. In User Profile Service, Click on Configure Synchronization Connections
10. Create a new connection to your domain
11. Enter a name for your Connection Name, Account Name and password to create a connection to Active Directory to import profiles to SharePoint 2010. Don’t forget to enter Forest Name
12. Once you have al the information, click on Populate Containers. Select the containers which you want to import to SharePoint 2010
13. Click OK to start Start Profile Synchronization. It takes time depend on number of users you have in your forest.
Aryan Nava 
Am unable to start the synchronization services from central admin–> when i try to start it is throwing error.