Hey Photo_Wed,
There are a lot of terms to lighting that is for sure, but lets start off with the basics:
Your car as of now has halogen light housings. The beam of light gets shot out of the bulb to that cap looking thing in the light. Then it gets reflected off of the mirrors on the inside, and shot out into the world.
The next thing you need to know are projectors. Projectors are on BMW's and Audi's. They are in the light, and look like a crystal bulb. That is actually how the light focuses on these cars, through this bulb.
Retrofits are where you take your stock headlight housing, put it in the oven to heat up the glue, and pull off the the clear plastic from the back end. Then, using that backing that you just peeled off, you buy another car's projectors and with a few cuts and slices, you insert the projector in your cars headlights. Then you close it back up by heating it up again.
There is the retrofit lol.
Now, you are probably saying...all this work for this plug and play, easy, lighting mod????
Unfortunately, yes and no. You could just get the kit, and install them in the same position and see what happens. 2 things could happen. 1. Everything works fine, and you may just have to adjust the position of the light. 2. You may need minor mods for it to fit. (Cutting that little cap to accommodate the new bulb.
The reason why people are saying to retrofit is because these lights are 3x more powerful that your standard halogens. Since halogens use a reflection type lighting, those more powerful bulbs would have so much light that you wouldn't be able to focus it. The other users are talking about glare, like having your brights on all the time.....no a better example were if brights had brights (better example

)
I was actually just going through this phase, and I ended up talking with a guy named Daniel Stern all about this. I got the philips Xtreme Power!!! They are the best halogen on the market right now...
You could get xenons and see what happens. Some cars are better than others with these plug and plays.... Id strongly recommend the xtreme powers though.
If you have questions just ask!