It was essentially like having a text-only version of the web built into your TV, but back in the 80s. TV channels would broadcast the data for the pages in the gaps between video frames and a TV with the appropriate decoder would cache the data and use it to build up the complete set of pages, which you would then access by pressing the "text" button on your remote control. Each TV channel would broadcast its own Teletext service, with most channels offering a mixture of news, sport, entertainment etc.
It sounds (and looks) primitive by modern standards, but it was actually a wonderful technology that worked perfectly well for its intended purpose and was very information-dense. Sadly, we lost Teletext in the UK during the changeover from analogue to digital TV, but I think it's still alive in a number of countries.
It was possible for a TV channel to encode a small amount of data in the vertical blanking period between video frames - this is how subtitles were transmitted, for example. Teletext exploited this feature by encoding entire pages of text into that interval, each with a three digit page number. When the user requested a particular page number through the remote control, the TV set would wait a few seconds until the corresponding page was transmitted, then render it. More expensive TVs would have enough RAM to cache the pages ahead of time, so page access would be instant.
It sounds (and looks) primitive by modern standards, but it was actually a wonderful technology that worked perfectly well for its intended purpose and was very information-dense. Sadly, we lost Teletext in the UK during the changeover from analogue to digital TV, but I think it's still alive in a number of countries.