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

Backup of an individual database, Web application, or the entire farm Using Powershell command

Backup-SPFarm -BackupMethod <String> -Directory <String> [-AssignmentCollection <SPAssignmentCollection>] [-BackupThreads <Int32>] [-ConfigurationOnly <SwitchParameter>] [-Confirm [<SwitchParameter>]] [-Force <SwitchParameter>] [-Item <String>] [-Percentage <Int32>] [-WhatIf [<SwitchParameter>]]

Or

Backup-SPFarm -ShowTree <SwitchParameter> [-AssignmentCollection <SPAssignmentCollection>] [-ConfigurationOnly <SwitchParameter>] [-Confirm [<SwitchParameter>]] [-Item <String>] [-WhatIf [<SwitchParameter>]]

BackupMethod - Specifies the type of backup file to be created.If a backup is performed with the

–ShowTree parameter, then the BackupMethod parameter is not used.

A full backup is a backup of all the selected data. A differential backup is a backup of all the selected data that has changed since the last full backup. If an item does not support differential backup, then a full backup is performed for that item instead.

The type must be either of the following:

- Full

- Differential

Directory  - Specifies the path where SharePoint 2010 Products stores the backup package it generates. If you have a computer on which Microsoft SQL Server 2008 and an instance of SharePoint 2010 Products are installed, you can use local drive paths. This includes a basic installation. However, if SQL Server 2008 and SharePoint 2010 Products are installed on multiple computers or if you have multiple servers running SharePoint 2010 Products, you must use Universal Naming Convention (UNC) share paths so that the SQL Server database and search components are written to the same location; for example, \\computer_name\volume\Backup).

Multiple backup packages can be stored in the same location. This is the same path that you pass to the Directory parameter of the Restore-SPFarm cmdlet.
The type must be either of the valid paths:
- C:\folder_name

Other parameters are optional .

In the Second command

ShowTree - Displays which objects in the farm will be backed up based on the other parameters passed to the Backup-SPFarm cmdelt, namely the Item parameter. Items that will be excluded from the backup based on the other parameters passed to the Backup-SPFarm cmdlet will be preceded with an asterisk character (*). Items that cannot be backed up will be enclosed in square brackets ([ ]). A backup will not be performed if the ShowTree parameter is present.

Source - http://technet.microsoft.com/en-us/library/ff607881.aspx