Both translate to "should". I understand "sollten" is the past tense of "sollen" but sometimes I see "sollten" when I would expect "sollen."
Since you mentioned you know that sollten is the past tense (Präteritum) of sollen, I'm going to focus on the difference between the indicative and the subjunctive.
The best way to understand the difference between sollen (present) and sollten (subjunctive, called Konjunktiv II in German) is to translate them as "have to" and "should", respectively.
Die Kinder sollen im Haus bleiben. - The children have to stay inside.
Die Kinder sollten im Haus bleiben. - The children should stay inside.
sollen sounds quite a bit harsher, basically an order. You could even translate the above sentence as "The children shall stay inside." (and "shall" is indeed the indicative form of "should") but this form is not really used in the English language anymore and sounds quite old-fashioned. It can still be found in books, though.
sollten is advice or recommendation.
Note that this is a very basic explanation. Apart from "should", sollen can also be used to express hear-say in indirect speech (e.g. Ich habe gehört, dass er jetzt irgendwo im Ausland leben soll. - I heard he lives abroad.) but this is another matter. I mention it only for completeness sake.
edit: I'm not sure how well you understand German, but there are some nice examples and explanations of sollen on this website: Canoo.net - sollen
If in present tense:
du sollst = you should (more an instruction)
du solltest = you rather should (more a recommendation)