A standard text message is 160 characters long. If your message exceeds the limit, an alert is shown on the screen allowing to continue typing the message beyond the character limit. This message will count for 2 or more SMS Credits (depending on the length of your message) and you will be charged correspondingly. Special such as ~, ^, {, }, [, ], |, will constitute two characters.
Bulk / Text SMS allows you to send longer SMS of upto 918 characters which are split into smaller SMS and later joined at the receiving end which is a mobile. In this scenario, SMS length is calculated for 153 characters as 7 characters are used to concatenate the message when it is delivered to the handset.