A rebuilt transmission is a transmission that has been taken apart, thoroughly inspected, and has had all its faulty parts replaced. Once the worn parts have been replaced, the transmission is put back together to factory specifications.
There is a huge difference between a new and rebuilt transmission. Drivers cannot buy a new transmission. They are only available for inclusion in new vehicles and this goes for those purchased at a dealer as well. Each transmission purchased from a dealer is a remanufactured, or refurbished transmission, so drivers should be wary of a transmission being sold as "new."
When a transmission is taken apart, technicians look at things like the seals, gaskets, bands and clutches. They replace what needs replacing and put the transmission back together.
A remanufactured transmission is basically the same thing, but in terms of the difference between rebuilding and remanufacturing a transmission, there is one subtle difference. Remanufactured transmissions are rebuilt in a factory and are worked on by several different people, all of which have a specific function, rather than by one mechanic or technician. One person tears it down, one inspects the damage and several are in charge of putting it back together. Most transmissions bought in a shop fall into the remanufactured category.