Should be in
Program Files>Microsoft SQL Server>MSSQL 1.0>MSSQL>BACKUP>
In my case it is
C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\Backup
If you use the gui or T-SQL you can specify where you want it
T-SQL example
BACKUP DATABASE [YourDB] TO DISK = N'SomePath\YourDB.bak'
WITH NOFORMAT, NOINIT, NAME = N'YourDB Full Database Backup',
SKIP, NOREWIND, NOUNLOAD, STATS = 10
GO
With T-SQL you can also get the location of the backup, see here Getting the physical device name and backup time for a SQL Server database
SELECT physical_device_name,
backup_start_date,
backup_finish_date,
backup_size/1024.0 AS BackupSizeKB
FROM msdb.dbo.backupset b
JOIN msdb.dbo.backupmediafamily m ON b.media_set_id = m.media_set_id
WHERE database_name="YourDB"
ORDER BY backup_finish_date DESC