When viewing reports we display both outbound messages and units used:
Outbound Messages
The number of messages which have been sent within the specified date range. One message maybe made up of several message parts (depending on the length of the message), but will be received as a single message on the recipient's handset.
Units Used
The number of message parts that have been sent within the specified date range. One message can be made up of multiple message parts when a message exceeds 160 characters (GSM charset). So a message of 161 characters will equate to 2 units. For multipart messages, the units are then calculated in blocks of 153 to allow for concatenation (so the message is displayed as one long message on the recipient's handset).
If you include any non-GSM character in your message the character limit is reduced to 70 for the first message and blocks of 67 for multipart messages.