'Te quiero' is a lighter way to say 'Te amo' (some could say) . For example, if you are seeing someone and you are at the beginning of your relationship with this person you can start saying "te quiero" until you feel something bigger than that, then you are ready to say "te amo" ...supposedly it's because "te amo" is a greater feeling than just "querer" ... well, I am not a fan of measuring love, but this is how those expressions work. ^^ ( http://www.italki.com/answers/question/70201.htm )

Ambas palabras indican un hermoso sentimiento: el amor. Solo cuando el amor es intenso e implica una gran pasión, decimos a alguien TE AMO.

Te amo es mucho más que te quiero. Es lo mismo pero en mayor grado. Es querer en demasía.

Te quiero is used for friends , and family (-) Te quiero is used for news couples (-/+) Te amo is used for couples , and lovers etc.(+)