you use the konjunktiv if you want to express something theoretically, a possible event. it is like the if-clauses, there are different types. depends on whether you express something that could really happen (1. example: Es könnte heute abend regnen.), something that is impossible (2. Ich könnte die Welt beherrschen.) or something that is impossible because it is too late (3. Ich hätte dir geholfen, wenn du mich gefragt hättest.). it is a bit more complicated I think, but that is the basic.
for further studyin look here
Konjunktiv I: (1. and 2. in my examples)
http://www.mein-deutschbuch.de/lernen.php?menu_id=36
Konjunktiv II: (3.)
http://www.teachsam.de/deutsch/d_lingu/synt/wort/Verb/verb_modus/verb_modus_3_3_3_2.htm
hope I didnt tell you something wrong, because it is 2 a.m. and that was long ago that I practised the konjunktiv - rules ^^