This is the entry level solution and will cost you <£10 more a month than your usual mobile contract (Most Blackberry contracts have this prebuilt into the contract already) With BIS you are able to forward your work emails to your Blackberry device and send and receive from that device.
If you want your calendar appointments or contacts then they will synchronise when you plug your Blackberry into your computer.
BIS is limited to push email. Calendar/Contacts/Tasks can only be done through a local sync using a usb cable. Sending and receiving email is the limit of BIS, for the full sync capabilities BES/BPS is necessary.