The Units tariff has two main parts to the ongoing charges: A monthly charge for the service itself, which depends on the type of line, and a number of pre-paid usage units that you buy each month.
The usage units are used up when you receive anything from the internet. Upload does not count. The speed that the units are used up depends on the time of day and also on the type of line you have. One unit is at least one gigabyte of download during the day.
The Unit tariff also includes a free .uk domain name and associated services (Up to 1G of email storage and 1G of webpage storage).