Here's some defined terms to help you along if you get lost. I'll throw them around without thinking, but I'll do my best to cross-link when I think of it.
CAD = Computer Assisted
Drafting/Design. The process of creating a 3D model of an object on the
computer. It's like a drawing or a set of plans. A CAD file is a
computer file that contains this information. There are as many file
formats as there are CAD software programs. Some of them are compatible
and some are not. This is a source of headaches.
.STL = STereoLithography file. A very popular file format that many 3D printers can read.
Most 3D printers can't read CAD files directly, so you have to convert
them when you are all ready to go. This is usually very easy.
The Build = the thing you are making, and the process of making it. "Yawn, I'm still waiting for the build to complete."
= the thing that the build is attached to as it's worked on. The build
pad holds the build in place, otherwise it would slide around and turn
out rather badly.
Supports = These are the little pegs/posts/pins that hold the model to the build pad, and keep the build stable and from falling apart while it's built. These are usually generated by software, and can't be easily modified.
Multiple Shells = a CAD term for a model that's not connected properly. It doesn't happen in the real world. A CAD model may look like it's attached to itself everywhere, but sometimes the models masses don't quite meet each other by a very very small amount. It can be difficult to detect, until you 3D print it and it falls apart. It can cause a big problems with supports, since the computer will think these are all unique objects and try and support them independantly.
Watertight = is a CAD term for models that are completely enclosed and that have no holes to the "interior" area. The interior area being the part that you want to remain solid. Here's a technical explanation. But in layman's terms, imagine you are filling the solid area with water. If the water can come out, it's obviously not watertight. A model that is not watertight confuses the printer, since it no longer knows what's inside and what's outside. These problems are usually found at corners when a model objects surfaces don't meet properly.