Why there is no difference between this two words - zakat and sadqah ?

I really want to know the meaning of the Arabic word "زكوة" mentioned in the Holy Quran(root - زكو). One replied to me the meaning is alms. Moreover, the word "صَدَقة" points to the same meaning - alms. So what is the difference between those two words? If the meaning is same then why Allah Subhanahu wa ta'ala mentioned it by two different words?