There are different ways that you can take backup of a site collection in SharePoint 2010 like stsadm, powershell or through central administration. First we will see how we can take backup through central administration.
First open your SharePoint 2010 central administration, go to Backup and Restore section and click on Perform a site collection backup. Then it will open the Site collection backup page like below figure. There choose the site collection for which you want the backup and give the file name and path in the file location and click on Start Backup.
Stsadm:
Here is the command to take backup of site collection:
STSADM -o backup -url <SiteCollectionURL> -filename <filename>
Example:
STSADM -o backup -url http://your-site-url -filename c:\backup\MySiteCollection.bak
PowerShell:
Here is the powershell command to take backup of a site collection.
Backup-SPSite <SiteCollectionURL> -Path <filename>
Also there are lots of parameters of this command.
Example:
Backup-SPSite http://your-site-url -path c:\backup\MySiteCollection.bak
You can check how to restore a site collection from the backup file.