수집 is a noun hanja word that means 'collecting certain materials for a purpose'. It can be also used as verb, like in 수집하다. 수집하다 means 'collect certain materials for a purpose' 모으다 is a Korean verb for 'collect (something with or without purpose)' ex) I collected garbage. 쓰레기를 수집했다 is awkward 쓰레기를 모았다 is OK. I collected stamp. 우표를 수집했다 is OK. 우표를 모았다 is OK.