Parametric modeling refers to the underlying method in which an object is designed in CAD. Typically you start sketching a primitive (block, cylinder, sphere, etc) that has dimensions that are parameters, meaning that they are editable later on. Next you add more features on top of the primitive, be they more extrusions, cuts, holes, rounds, chamfers, etc. All these features are built in a tree structure, which could number in the many hundreds for real manufactured parts. These features have very complex parent/child relationships based on references which could sometimes be very problematic if you edit or delete a feature, you could damage all its children features if they cant find their parents. All these features are defined with their own unique dimensions with a unique parameter id. The basic idea of parameters is that you sketch out the basic shape quickly first, then worry about the specific sizes of things later. This concept is incredibly powerful because it gives your design flexibility, or “strechyness”. For example, you sketch out a plate 100mm by 100mm by 10mm, then put a grid of 5mm holes spaced every 20mm. With parameters you can make it such that later on you can increase the size of the plate to 200x200 and all the holes will strech out evenly, proportionally, instead of remaining in a clump in a random spot on the now bigger plate.
Associative design typically refers to the same concept of parameter but across different part files that are put together in an assembly. So for example if you edit the spacing between a series of holes in part A, then the matching threaded holes on part B will automatically adjust their position to match A.
The parametric concept was first popularized by PTC with their product Pro-Engineer back in the 80’s, and beat the pants off their competition until many years later they were able to catch up. In the last five years there has sparked another great design paradigm shift where parametric is replaced by synchronous modeling. Siemens PLM with their flagship product NX (fka Unigraphics) is leading the way with this new concept, which is, well, extremely cool and versatile.
Contrast all this with non-parametric modeling (such as AutoCAD) where you have to sketch all your lines and geometry with the correct dimensions from the beginning. Good luck if you change your mind and need to resize things later on. You have to delete what you sketched and draw the new things in their new place. You might as well be using pencil and napkin, it’s so useless.
After using parametric CAD, you will want to gouge your eyes out with a cheese grater if you attempt to use something non parametric. Likewise if you work with something with synchronous modeling (or active modeling), you will want to lobotomize yourself if you are forced to use something without synchronous.
So there you have it. The cutting edge open source CAD packages out there are basically 30 years behind the curve (not an exaggeration at all 
). So I think you will realize sooner or later that the initial investment will pale in comparison with the time investment learning anything.
