Main effects:

The effects of those variables you are able to control easily, such as machine speed, amount of a component in a formulation, and stirring rate. These variables are often, but not always, reasonably well understood.

Interactions:

Suppose you have a good understanding of how two variables affect the quality of your product, but when you make changes to these variables at the same time the effect is much greater (or less) than that expected. This is an interaction. For instance, in an adhesive both resin and hardener need to be changed to see a significant effect, since the levels of these components interact. Interactions are usually not well understood but can be very important. Technologists often have a feel for which variables need to be altered at the same time to get a particular result, and it may be worth quantifying these interactions.  For more information click here.

Noise:

Those variables that are difficult or impossible to control that affect the process. These are often environmental such as ambient temperature and humidity, but can include batch-to-batch variation of raw materials and the use of different operators and machines.

Robust:

A process that does not change with changing noise. This is an important part of the Taguchi technique, and is often investigated using a parameter design.