Encrypted channels are deliberately scrambled, so that a special decryption device is needed to watch - or listen - to programming. In analogue technology, encrypting a television channel is mostly done by putting the video lines into a different order.
Digital transmissions can be encrypted in a wide variety of ways. Usually, a special card (similar in size to a credit card) has to be inserted into a satellite receiver, often in conjunction with a Conditional Access Module (CAM). This card contains the code to decrypt signal.
Those cards are only available from the programmer, in most cases at a price (pay-tv). In some cases the cards are available free to certain citizens only, to restrict an otherwise free channel's availability to certain geographical areas.

Here is a list of encryption method abbreviations:

ACGA AccessGate BCNV Betacrypt/Nagravision BCRT Betacrypt BISS BISS COCN Conax/Cryptoworks/Nagravision COCR Conax/Cryptoworks COMG Conax/Mediaguard CONV Conax/Nagravision CONX Conax CRYW Cryptoworks CWVA Cryptoworks/Viaccess DMV_ DMV DRMC DreamCrypt IRCW Irdeto/Cryptoworks IRDT Irdeto IRKF Irdeto/KeyFly IRMG Irdeto/Mediaguard IRNV Irdeto/Nagravision IRVG Irdeto/Videoguard MCV_ Mediaguard/Cryptoworks/Viaccess MCVI Mediaguard/Cryptoworks/Viaccess/Irdeto MDS_ MDS MGCW Mediaguard/Cryptoworks MGIV Mediaguard/Irdeto/Videoguard MGRD Mediaguard MGVC Mediaguard/Videoguard/Cryptoworks MGVG Mediaguard/Videoguard MGVI Mediaguard/Viaccess/Irdeto NAGV Nagravision NTL2 NTL 2000 POVU PowerVu PVU+ PowerVu+ RAS_ RAS SKYC Skycrypt TASC Tadiran Scopus TVCM TV/COM VC2+ VC2+ VCIR Viaccess/Irdeto VCON Viaccess/Conax VGRD Videoguard VIC_ Viaccess/Irdeto/Cryptoworks VICS Viaccess VIMG Viaccess/Mediaguard VINA Viaccess/Nagrvision VIVG Viaccess/Videoguard WGNR Wegener