Encryption can be as basic as replacing every 'a' in the plain text by a 'b', every 'b' by 'c' or, easier for computer users, by hitting the key right or left to the original key (I'll give an example for a german notebook keyboard):
(plain) TRAP17 IS GREAT
(a->b) USBQ28 JT HSFBU
(right) ZTSÜ28 OD HTRSZ
--the fourth letter in the last example is a german Umlaut, a special character. It might not be displayed correctly in other regions--
Well, that's encryption for you. It is, however, not secure since the pattern will repeat and thereby it will become easy to crack. Some smart people invented a secure (well, as secure as encryption can be. If you lose the secret key, it's not secure anymore but hey, that's the human factor) encryption method working with a pair of keys, one of them public and one private. Unlike other encryption methods, public key-encryption works better the wider you can spread your public key.
Whoever wants to send you an encrypted message uses your public key to encrypt it - but this message can not be decrypted using your public key. Your private key on the other hand enables you to decrypt it.
Using this method, a text (or file) can even be "signed" by you - use your private key to encrypt the text or create the text's signature and it can be decrypted/validated using your public key...
...this method is used in the proprietary PGP("Pretty Good Privacy")-Software and the compatible, free Open- and G-PG("GNU Privacy Guard")-Software and some other software to encrypt your communication.
If your private key-file falls into the wrong hands, however, you should be quick to spread your revokation certificate (a file PGP/OPG/GPG can generate using your private key) so people know for sure that your private key is not private anymore (well, it might still be secure but everything that prevents someone from using your private key to decrypt your communication is a simple passphrase like you can set for e.g. a web service)...
by the way, "TRAP17 IS GREAT" looks like this when encrypting it using GPG and a 4096bit-key (i.e. the one I'm mostly using):
QUOTE
-----BEGIN PGP MESSAGE-----
Version: GnuPG v1.4.3rc1 (MingW32) - WinPT 0.11.8rc1
hQQOAy8jq7axk+gwEA//V6Uj5gZIbggG5YaWA5nbM075gGobq168gxq1Arj03Zu6
RXHdYu8ZZOqqGlnhkGkkXgvlKLuCRNp00t0JjjWAR7PBgQYewM9bLX4WXNju94ju
OQ2StfS5fL/kGIYihkEseuD0lojdIXLFJj3Urx39L6QaFSwfy+V0H0JIXk8Wp08k
2XVaZPsZDpzArNjEnb09xsIzzY/HMDLjcMbh/KIXw8TyytW4RZ7Mapdaba4Layaj
8bZTMzNpASP1dZkVTle+S4as5DM4Coam80VjGh3DRtK/khVcivW09rybydgAX5U2
KIbz1r6o0uCpjOWr6YcHF7eKwjQ54QvD4N7Y20yA9K8pN7JlnvYAUfJ9YkBnD9An
pjNwjITLRl3dJtaKt/3lR7RBWnwmu7xiZ9SbF1PSJU1IND/Vh9heCUEJfNCi2dIH
ECjjz7kYh8ff6tWD8ZO1QHmIbU+JWWauRBvunT7W9GPCNjj42UP+mf79M4AHEGc9
Tlc6E0yiv5Ud+OFzPGAMDTMtwR85Psk+FuVnUjnLD5oVnTjWX1Zv2Nq5t2N8zKLu
J8gqCW7PnKLxtk8bgw88kO1gsbe/iO80HX9qY9AMx7gPl9Sk7PuWj6WG6ykVG9gg
YxBDQoipK55WiLas9Eh6gRH7WiwRI3f8Ixcm4a8FA7tleDgNEhzmfN4YeRn+oMIQ
AKbtPc5M37HrgN8086/0Rd/U8OoLRhpFKT9SeaoBLWDRMf1TvlwEcCmKWZ4itSMp
Hk6yofeQq2HBhsiUgJ3jw29tF0qI4h+2LCCGF+2U6vcgIQWTj6wKuZsQfh/MlkT9
cy7NPd3PDjbW2wMOyHYN6KUVpFGxTqStfvvD5LMBA/XAgSD/rMsnzNgjhy65B+SI
qu4zL3CV47Rc/PihciZNooZUuRWEOXrfd15s9s4UXMpqVwL8hfFE4CEu5WLfEAfZ
P4g7MrV3pE+SobVXfjP8xqK242Xe6sxzAnzLEIvA+jWuz8UtI/1uQJ2DE62o1CIn
rqCkLUilQGlJABtAqXujDNGWNDNJv9RDTQ9eXRdudEfMJDjatkRbUAxmEFccGZrX
biG2IeKQHRRN5qI6t90uylojwsFZbV8OrDc8JftMEapxPxM/qC87c+NK1PAo6LZ8
jrMgvioDsuUcxH2ubFxE+fnA31TPW/sDD/X6xo7OOsnrvbjc35i9rKsOeiWbwYnQ
vRv4Lh4TH8naozWA0glofug1SDDoUzcFyTx0bKPjHDtbqHxaQawW+3Kr56V7ECsk
PvSe1rGnxNzWf55Rr6k4bjzYT+aOM58GAYsYCCywzgPnzsqBS05ZXIPCexb6gswQ
ba1vCYIkj6OkNpVoXUyqtfwrZBXHyH9ODfz2oajmDfz70koBOeEEuMXNbd0QX/Yy
r0wWhW8ERImnfBoryJ81kPybwIya0vkimzoCbEFEugESbCQfMBKRigSvaNnWkxhl
6I9Ae6WflsSgSCOJ3Q==
=vib9
-----END PGP MESSAGE-----
try guessing that...
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