How is data stored on computers

When you turn a computer off, it forgets everything that's not written down. Computers write to a "storage device" or "block device". This can be a hard disk, and SSD, flash memory, even a floppy disk. The data is stored in a logical organization scheme called a Filesystem. Writing a new filesystem to the device or medium makes the old data inaccessible- but the old data is still there, and a determined person can find it.

We want to get rid of this old data, and that means writing new data over it, or *overwriting* as it's called. The old data needs to be overwritten several times with highly entropic patterns in order to best obscure the old data.

Risks of having data stolen

Identity theft
Monetary loss
Public embarrassment
Criminal prosecution

How to protect your data while in use via encryption

App encryption

GPG / Seahorse for general on-the-fly encryption / decryption of data, signing and verification, key management

CryptoCat / OTR for encrypted or anonymized chat

Full disk encryption

LUKS
TrueCrypt
ecryptfs

What to do before handing your hard drive to a recycler

(e.g., you've upgraded to a new hard drive or have donated your old computer).