From aa-asterisk.org.uk wiki
The Digium Asterisk Hardware Device Interface, or DAHDI, is the new name for the Zaptel drivers that are supplied with Asterisk. The reason for the name change is that ZapTel own the trademark and therefore Digium's drivers needed to be changed.
DAHDI (and Zaptel before it) drivers are designed to drive various hardware devices such as Digium's analogue and digital telephony interfaces, but are also used as the basis for cards made from other manufacturers such as Sangoma. Sangoma's WANPIPE drivers are a derivative work of these drivers.
DAHDI drivers are also used to supply timing to Asterisk for TDM devices. Where a TDM hardware device is not present, the ztdummy driver may be used which uses the PC's real time clock for timing. Meetme requires this to operate properly.