A videotex service available over telephone lines rather than with the broadcast television signal. This allowed interaction.
In the UK, the frame format was the same for teletext and Viewdata - 25 lines of 40 columns. Lines 2 to 23 were for frame data. Line 1 contained the page heading - the IP banner, page number (or name) and the page price. Line 24 echoed user input. Line 25 was the local status line.
The teletext node provides most of the information regarding the display format. The transmission format encoded the control information as ESC followed by a letter (A for red text, B for green text, M for double-height, [ (that's left-arrow, btw, not open square bracket - Viewdata had a different character set to the PC) for new background colour).
Unlike teletext, Viewdata allowed the user to enter a full nine-digit page number. You pressed *, then the number, then # ("square" in BT parlance). Within a page could be up to 26 frames, named "a" through "z", accessed by pressing #. In a similar stroke of genius to the telephone system, no page number could begin with zero, so *0 sequences had special meanings. *00 redisplayed the current frame (without refreshing the data or incurring a further frame charge - basically, if you got hit by li}e no!se, you hit *00 for another go). *09 redisplayed the frame, but refreshed it and incurred another charge (e.g. get the latest share price information). *0# didn't take you to page 0 (well, not on a decent host) but to your own main index. *# was the equivalent of the browser's "back" button.
In addition, there could be up to 10 "links" on a frame, accessed by pressing 0 through 9. These could go to any other page. A well-crafted site used them consistently (e.g. 0 for main index, 9 for index of current article, 7 for previous section/article, 8 for next section/article and 1 through 6 to select an article from a section index).
The first UK provider of Viewdata was the GPO (the General Post Office), which shortly got split up into the Post Office and British Telecom, who kept the Viewdata service. The service was called Prestel, founded in 1979.
IIRC the monthly subscription, including Micronet (the interesting bit about computers) was pretty hefty - something like £12 a month(?).
On Prestel, the page "namespace" was split into magazines in a similar way to teletext - the first three digits identified the magazine. It was then up to the Information Provider to control the numbering at lower levels. (So, *100# was the main Prestel index. *800# was Micronet. *800800# might be information about Micronet. Some IPs were more rigorous than others.)
Another advantage over teletext given by being interactive was being able to post messages. Micronet had "realtime" "chat" rooms! The user filled in a form and the message content was copied onto a particular page.
Later services, such as The Gnome at Home, used full alphanumeric page names. So you could have *BBS.0# for the bulletin board pages index; *BBS.1# for the first bulletin board; *BBS.1001# for the first article on the first bulletin board. (On the BBC Micro, under ADFS, the dot was a directory separator. It is left as an exercise for the reader to guess how these page names mapped to the file system...)
Other heavy users of Viewdata were Travel Agents. You may still find viewdata displays in many UK travel agent's shop windows.