University Challenge has been running on British TV for 53 years and usually consists of a big competition between teams of students from most of the the country's Universities. The Christmas special maintains the format but there are fewer teams (in this case 14); the teams are picked by the producers; and the competitors are graduate alumni of their institutions, considered to be notable in some ways.
The appeal of UC is that whilst the format is simple, the questions are devilishly difficult to answer under pressure. The producers say that the best of teams typically only get about half of the answers right. People watching at home enjoy watching teams struggle and are ecstatic when they themselves know an answer to a question that foxes the players.
Personally, I was terrified at first by the invitation to take part. Then I decided that it would be fun, whether we got anywhere or not. And my team mates were lovely people to work with.