No, it's not your fat training partner!

A "Tubbie" is a tyre incorporating a tube which is sewn together and then glued onto the rim.
Positives: 

• Higher tyre pressures can be attained (up to 10/12 bar, where tyres are normally 8 bar).  This results in less rolling resistance and more responsiveness.
• Speedy puncture change on race days

Negatives:

• More costly