Backup and Restore using stsadm.exe

You can backup & restore using below ways.

1. Using STSADM.exe command.
2. From Central Administrator Backup & Restore.

Backup - Restore using STSADM command
Open Start > Run > type "cmd" OR "command"
then type "cd C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\BIN"
Then you can use command as per your requirement.
Backup Site Collection
stsadm -o backup -url http://webapplication/SiteCollection/" -filename "C:\backupFolder"
Restore Site Collection
stsadm -o restore -url http://webapplication/SiteCollection/" -filename "C:\backupFolder"
Backup Farm
stsadm -o backup -directory "C:\FarmBackupFolder\" -backupmethod Full
Restore Farm
If you want to overwrite existing farm use stsadm -o restore -directory "C:\FarmBackupFolder\" -restoremethod overwrite
If you want to restore new farm use
stsadm -o restore -directory "C:\FarmBackupFolder\" -restoremethod new