3D Printed models are not usually solid or hollow inside. They contain "infill". The infill is placed in a pattern which can be adjusted depending on the options of your specific printer. The percentage of infill can be increased to create a denser and stronger model. But increasing the infill will also increase material usage and print time. These photos show examples of infill percentages and infill patterns from our Makerbot Replicator. The infill shown in the picture below is in a "diamond" pattern at 5%, 10%, 20% and 30% infill.
This picture shows two other patterns of infill available on the Makerbot Replicator - Linear and Hex.
The "Shells" are the number of layers of material on the outside wall of the model. Increasing the number of shells adds to the thickness and strength of the model, but also adds to material usage and print time. The default setting on the Makerbot Replicator is 2 shells, but this can be adjusted. See examples below of 2, 3, 4 or 8 shells.
A Raft is a thin latticework of filament beneath your object. It is a foundation for the object to be printed on.
When the print is finished, the raft is removed.