Good Job!!! you actually did research of you own and didnt just take my word for it. For that you get kudos.
ok a spring is just that a spring
http://www.overboost.com/obs/product/eibach_prokit.jpg . you put it on the strut, aftermarket springs are shorter which lower the level of the car cause thats what the car is sitting on.
with that spring you'll want a better strut
http://www.overboost.com/obs/product/tokicohp.jpg cause the stock ones will wear out fast
a coilover is a spring with an adjustable sleeve incorporated into it
http://www.overboost.com/obs/product/gc_4.jpg you can lower/raise the spring on the sleeve which lowers/raises the vehicle.
now there is another coilover a REAL coilover which is a threaded strut with a matched spring
http://www.overboost.com/obs/product/jic_flt-2.jpg
you adjust the height on the strut instead of on some cheap sleeve.
did that clear things up?