Taking back up of site collection in sharepoint using power shell

Run SharePoint 2010 Management Shell as Administrator

Enter the command in following format : 

Backup-SPSite http://server_name/sites/site_name -Path C:\Backup\site_name.bak

Example : 

Backup-SPSite http://inpunects01:7676/sites/ITServices -Path C:\Backup\ITServices.bak

You can name your bak file as per your wish.I have named it ITServices.bak

Dont be worried if you go to this path and find a small bak file,keep refreshing this folder and you can see that the bak file size keeps on increasing as long itdoes not take complete back up.

I would suggest,run this command and go grab yourself a coffee,when your back your .bak file is Ready ! 

References : http://technet.microsoft.com/en-us/library/ff607901(v=office.15).aspx