2. Normal deletion Shift deletion
Usually, when we delete a file, it goes to the Trash/Recycle Bin, provided the file is not larger than the bin’s capacity.
One can restore these files easily.
Shift deletion
If SHIFT+DELETE is pressed with objects selected, such as files and folders, this will bypass the recycle bin and delete the selected objects permanently.
We may think the file is irretrievable — it isn’t. Only a pointer to the file is deleted as yet, not the data, which remains on the disk.

3. Dos and Don't that must be followed in case we have lost data:
Don’t OVERWRITE any data on the device.Shutdown the system immediately.
Do not install any new software (not even the recovery software).
Do not run any software(including the OS) on that drive.
DO'S
Reboot the system with a live CD or other live version of Linux running off other media, like a USB thumb drive.
Download the required recovery tools if not available in the live CD.

4. FILE DELETION-What happens?

5. In both Windows and Linux,
Data stored on a medium as files has a “table of contents” indicating the storage location for each file (name) on the drive. When a file is permanently deleted using Shift+Delete or an rm command at the command-line in terminal, its entry is removed from the table of contents but not the data.
The ones and zeros that make up the file are still there on your computer, just the path to access that file is removed .This indicates that the space previously used by the file can be used to store other data — it is now “available” space.
When a new file is written to the disk, and uses this “available” space, it replaces the previous data with new data (the space of the deleted file is overwritten). So, recovery of the old file becomes very difficult (though not impossible).

6. COMMON RECOVERY TOOLS

7. IN WINDOWS:
Recuva
Total Recovery
Restoration
Undelete Plus
Pandora

8. IN LINUX:
Ddrescue
Data Life Saver
PC Inspector
SpinRite

9. IN MAC:
Disk drill Basic
File Salvage
Tech Tool Pro

10. MULTI PLATFORM recovery tools
TestDisk is a simple but powerful tool for analyzing and recovering disk partitions, including some unusual selections such as Xbox and Mac partitions
PhotoRec is designed to recover lost files from your digital camera's memory or any similar memory device. It can handle most memory types, including USB, CompactFlash, MMC, and SmartMedia as well as optical disks and your hard drives

11. Concept on which recovery tools works...
Data recovery tools are softwares which are capable of restoring the lost or deleted files or folder, once removed from recyclebin.
We know that the file is not actually removed from the drive,just left unmarked in the Inode table.
Thus, the tool searches for the files having unallocated space and whose address is marked as empty space. Hence, it is capable of restoring files.