Perform a backup of a site collection Using powershell Command

Backup-SPSite [-Identity] <SPSitePipeBind> -Path <String> [-AssignmentCollection <SPAssignmentCollection>] [-Confirm [<SwitchParameter>]] [-Force <SwitchParameter>] [-NoSiteLock <SwitchParameter>] [-UseSqlSnapshot <SwitchParameter>] [-WhatIf [<SwitchParameter>]]
Identity -

Specifies the URL or GUID of the site collection to be backed up.
For example, a valid URL, such as http://server_name/sites/site_name or a GUID such as, "01234567-89ab-cdef-0123-456789abcdef"

Path  -

Specifies the full path to the backup file (that is, C:\Backup\site_name.bak.

Detailed Description

The Backup-SPSite cmdlet performs a backup of the site collection when the Identity parameter is used.

By default, the site collection will be set to read-only for the duration of the backup to reduce the potential for user activity during the backup operation to corrupt the backup. If you have SQL Server Enterprise Edition, we recommend that UseSqlSnapshot parameter be used because this ensures a valid backup while it allows users to continue reading and writing to the site collection during the backup.

Source : http://technet.microsoft.com/en-us/library/ff607901.aspx