It's not a modified hybrid as many people think. Compared to their older rad brothers CX bikes have slightly more relaxed geometry and higher bottom brackets, slightly longer wheel base... in esence, it's a modified road framethat can take some serious off-rad beating. CX was developed as a bike for winter seasaon for road cyclists. However, over time it evolved from strictly an off-season training rig to a category of its own.
Hybrid bikes are usually menat for city and ocasional rding on the grass and very easy off-raod. Hybrids are not meant for racing, their geometry is even more relaxed and often the frames are not state of hte art as with CX, road and MTB bikes. MTBs are agai a category of its own. They are very different because theyis sole purpose is riding and racin on off-raod (many CX races involve both road and off-road riding). MTBs also come with a ton of different features, purposes and geometries so hat CX really looks much closer to a road bike than a MTB (and in essence CX is really a modified road bike for ocasional off-road use).