The danger increases if you have both plain-text and encrypted versions of similar or identical files on the disk. Merely deleting a file is no protection. Even a computer that's been through a flood is salvageable. Programs exist that will repeatedly overwrite files. But unless you take such steps, it's best to hold onto your old hard drives or, if you've stored secure data on them in plain text, smash them.
Decryption is the reverse process.
The whole category of products is referred to as encryption products. Even the sender can't decrypt it using the algorithm used to create the encrypted file. You can publish the encryption key, which lets anyone send you private messages. They later formed a company with the same name. RSA also refers to public-key encryption technology.