yes, there's huge difference to put it in simpler words, perfective and imperfective verbs are used to describe finished and unfinished work perfective Мы построили дом = We have built the house (it's finished, we are now able to live in it) imperfective Мы строили дом = We have been building the house (but for the lack of materials the building wasn't finished)

Hi rGama, this site gives you an excellent explanation with examples http://russian.speak7.com/russian_verbs.htm