Well, basically "Hatarakimasu" is a present form and "Hataraiteimasu" is a present progressive form. But when we actually use them, we use "Hataraiteimasu" as a present form and we say "Hatarakimasu" as when we want to refer to the future.
For example...
"I work as a teacher." when you translate this into Japanese, you might say 「わたしは教師としてはたらきます。」 I can't say it's wrong, but it just doesn't sound right. We say 「わたしは教師としてはたらいています。」
"I'll start working from tomorrow." As for this sentence, we say 「わたしは明日からはたらきます。」
"I'll work until 8 o'clock today." this translation would be 「今日は8時まではたらきます。」
hope you could get the idea of these differences.