TubeNet

In postings on this esteemed forum, reference has been made to "bass tubas" and "contrabass tubas". What does this mean exactly and how are these horns different from an ordinary horn?
Thanks!

Bass tubas = F & Eb tubas. Contrabass tubas = CC & BBb tubas

Thank you very much for your responses! This forum provides huge amounts of information in a very accessible manner.