Main difference between normal investing and algo trading is, when someone invests on their own, he/she could just follow some news and invest accordingly, invest based on SMS tops they receive, based on news article they read on newspaper, simply put there is no clear defined rules one follow and still continue to invest in markets.

But with algo trading you need to have some set of pre-defined rules. Once the conditions get satisfied the trades get executed . In manual trading you need to closely glue your terminal to verify whether these conditions get satisfied but in algo trading Machine does it for you