If you are using Windows you can use UNetbootin.

From Linux you need to use dd command that is pre-installed. But first download Debian or any other Image file that you want to save to USB stick.

Then put USB stick into computer

Then run command sudo dmesg. It will print something like that:

[21704.383412] USB Mass Storage support registered.
[21705.380872] scsi 6:0:0:0: Direct-Access     General  USB Flash Disk   1.00 PQ: 0 ANSI: 2
[21705.381737] sd 6:0:0:0: Attached scsi generic sg2 type 0
[21705.382457] sd 6:0:0:0: [sdb] 7831552 512-byte logical blocks: (4.00 GB/3.73 GiB)
[21705.382953] sd 6:0:0:0: [sdb] Write Protect is off
[21705.382956] sd 6:0:0:0: [sdb] Mode Sense: 03 00 00 00
[21705.383750] sd 6:0:0:0: [sdb] No Caching mode page found
[21705.383796] sd 6:0:0:0: [sdb] Assuming drive cache: write through
[21705.386078] sd 6:0:0:0: [sdb] No Caching mode page found
[21705.386119] sd 6:0:0:0: [sdb] Assuming drive cache: write through
[21705.387475]  sdb: sdb1
[21705.390321] sd 6:0:0:0: [sdb] No Caching mode page found
[21705.390362] sd 6:0:0:0: [sdb] Assuming drive cache: write through
[21705.390406] sd 6:0:0:0: [sdb] Attached SCSI removable disk

USB device name is sdb.

Then execute command:

sudo dd if=image.iso of=/dev/sdb bs=4M
sudo sync

Obviously replace image.iso with proper file name. And do not forget to execute sync when first command finish.

Here are the three commands you need to execute:

sudo dmesg
sudo dd if=image.iso of=/dev/sdb bs=4M
sudo sync

Write ISO image to USB