The EIT (Event Information Table) is extra data that is broadcast with a television signal, much like Closed Caption Text in the VBI stream. EIT provides program data for the current show and future shows which can be then used to view on-screen program information, such as title, length, description and more. This can also be gathered to build a program guide for current and future shows on all your available channels. MythTV uses an Electronic Program Guide, or EPG. Most cable and satellite services do. Even many modern digital televisions can have a program guide by gathering and storing EIT.
EIT is typically available over digital television signals, either terrestrial, cable or satellite. With terrestrial signals, each channel provides it's own EIT. Information is usually only available for the upcoming 24 hours. With satellite, the EIT is broadcast on it's own stream, or an Aggregate Event Information Table (AEIT), and contains information for all the channels broadcasted, and can provide scheduling information for more than a week.
It seems that for some people EIT only works when channels have been scanned by MythTV (this has been reported to have been necessary by a DVB-T user in the UK and a DVB-C user in the Netherlands). Importing channels.conf doesn't provide enough information for MythTV to use EIT.
(Note for Australian DVB-T users: Australian TV stations only broadcast the bare minimum Now and Next EIT information, (expect ABC which are trailing 5 day EPG) therefore there is little value in using this data with MythTV. Moreover, it is often a generic title such as "SPECIAL EVENT" or "AFTERNOON MOVIE.")
Tip: If EIT does not appear to show in the OSD, you can easily enable EIT in Mythtv 0.20 (assuming EIT is compiled in mythtv) using a channels.conf created from a program such as 'scan' (dvb-utils) import channels.conf into mythtv-setup, now delete the channels. This will now have populated mythtv with the correct transports for your transmitter/transponder (sometimes mythtv cannot find the transports on it's own). Now let Mythtv do a full scan using the existing transports. You should now have EIT showing in Mythtv. Users of DVB-T (Freeview) in UK will now have the next seven days of listings available (it may take some time for the listings to build up as the EIT is scanned).