A standard SMS message can contain up to 160 characters because ita uses the GSM alphabet. Certain characters (e.g. Cyrillic, Arabic or Chinese) are not supported in the alphabet. To be able to send these characters in an SMS we use the Unicode standard, Unicode messages are limited to 70 characters.
Wikipedia has a good overview of the standard GSM alphabet;.
Web interfaceThe web interface will automatically switch to "Unicode Mode" when you prepare a message with Unicode characters.