Ecto-1 is the name of the vehicle driven in the "Ghostbusters" movie, the name appearing on the license plate.
Aykroyd's design of the "Ecto-mobile" was to be an all-black, rather evil-looking machine with white and purple strobe lights. The effect was intended to be more of a hearse as a pun on the Ghostbusters' job. However, for practical considerations they decided to not go with the all black design because almost every shot of it in the movie was at night -- if they had left it black you wouldn't have been able to see it through most of the movie and thus it would have lost most of it's appeal and would look rather stupid.
The design that was eventually decided was based on 1959 Cadillac Miller-Meteor Ambulance. For added moxy they attached all manner of junk on the roof to give it the "Mad scientist" look: at least 3 police lightbars, a TV antenna, marine radome, firetruck siren, light board and even a giant air compressor tank were used in the finished product.
In fact, there were two ECTO-1's produced -- one primary and one to be used as a backup in case something happened to the original. However nothing did happen to the original and so it was seldom used for anything but promotional appearances. The rumour is that the secondary was, excepting the paint and props, quite a heap that could barely move without shaking itself to pieces and losing bits of the undercarriage to rust.