The difference is that "everytime" (1 word) is wrong and "every time" (2 words) is right. In sentences like, "Every time he calls, I'm never at home," you must spell "every time" as two words. As far as I know, "everytime" (1 word) is not a word and you should not write it this way.
Many people also have problems with "everyday" and "every day." When you want to say how often you do something, you must write this as two words: "I have coffee every day." "Every day, I wake up and take the train to work." Written as one word, "everyday" is an adjective that means ordinary. "He makes beautiful art with everyday materials like flowers and leaves."