Before you can print a model successfully, you need to know it is manifold with all the surface normals pointing out.
Non-manifold objects will cause problems even before you get to the printing stage. If you apply a boolean operation, eg to join two objects or subtract object from another, you will get the wrong result if one of your objects is non-manifold. Objects downloaded from the web are very likely to be non-manifold, especially if they were not built specifically for 3d printing.
There are various definitions:
My version is:
Or there is always the mathematical definition on wikipedia.
Alternatively you can try fixing the issues inside Blender. This tutorial by wickedwidgets goes into lots of detail on how to fix non-manifold objects in Blender. Its worth reading, even if you plan to use netfabb as your first choice. The first couple of steps are very simple and may be all that are required:
If the non-manifold area is an obvious hole, you can fill the hole. Select all the vertices around the hole, then fill. If the area is not an obvious hole, it may be intersecting surfaces or something similarly ugly. In that case, its off to netfabb or following the wickedwidgets tutorial to fix it in Blender.