From Memory Beta:
The Columbia-class was born out of the existing, and proven, NX-class design, and included retrofitted ships such as the SS Enterprise and the USS Endeavour. It was named, at the insistence of Admiral Jonathan Archer, after the first vessel of the NX-class to be lost in action, the Columbia.
As well as the refitted ships, eight new Columbia-class vessels were slated for construction: USS Buran, USS Shenlong, USS Apollo, USS Soyuz, USS Ares, USS Charybdis, USS Phoenix and USS Valiant.
The most noticeable difference between the NX and Columbia classes was the addition of a secondary hull which was slung underneath the primary hull. The secondary hull included a new navigational deflector, and also a new large warp core which allowed speeds over warp 6. The room in the primary hull released by the removal of main engineering allowed for improvements in crew accommodation, although the connection of the secondary hull did result in the loss of the main shuttle bay in favour of smaller drop bays.