Ok, here's what I can remember about the history. Bear mind some of this happened before I was even alive, so I might not be spot on.
Top Gear started in 1978, according to IMDB. The foramt was fairly similar since it's inception - a number of items, each dealt with by a different presenter. Some well known (in the UK at least) names were presenters, including Noel Edmonds and Angela Rippon.
In 1989 Jeremy Clarkson joined the programme. His distinctive style worked well, and along with Tiff Needell and Quentin Willson, the show was doing pretty well. This lasted until 2000, when Clarkson left to try other projects, including a chat show. The show carried on for another year until the BBC decided Top Gear had reached the end of the road (see what I did there) and didn't renew it for another series.
The fairly newly formed Channel 5 (now 'five', all lower case) bought the format from the production company, keeping the same presenters: Tiff, VBH (mmm), and Quentin, maybe some others too. This became Fifth Gear.
Then in 2002, the BBC realised getting rid of Top Gear was a pretty stupid idea. They got Clarkson back on board, and relauched the show with the same name, but a completely new (and enormously succesful, imo) format. All the other presenters were new faces. This is Top Gear as we now and love it today. Oh, but James May replaced the fat guy from Series 2 onwards.
There have been other motoring shows on terrestrial UK TV, Channel 4's Driven was pretty good in places. ITV's attempt, Pulling Power is, quite frankly, pants. I'm not sure it's on anymore.
Hope this helps.