It's BT's packet-switched digital network covering both phone and data.
Basically it means you can get up-to-24 Mbit/s ADSL2+ BB offering from ISPs using BT Wholesale's network, incl. BT Retail, Orange, Plusnet and other resellers. Phone (PTSN) has been shelved for the mo'.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC