An ADSL splitter separates the analogue voice-frequency signals from the ADSL (broadband) data signals(Broadband being defined as data transfer greater than 128KBPS.)
If ADSL splitters are not used, the ADSL data signals are heard as "noise" on any equipment connected to the "normal" telephone-line. Apart from being annoying during a telephone conversation, there is the possibility that the additional ADSL signals may cause problems with alarm-units (etc) that may be connected across the line.
