가지마~~

V+지마 means "don't V" in a very informal/casual way (반말).

It is used to tell someone not to do something aka give a command or order. The subject can only be "you" (singular or plural). It is only used to talk to and about the listener and what you are telling them not to do. It is used to form imperative sentences. "Don't blah blah blah!!!!!!!!!!"

Don't go to school!!!! 학교에 가지마!!!!!

Don't eat apples!!! 사과를 먹지마!!!!!

Go to school!!! 학교에 가라!!!

Eat the apple!!! 사과가 먹어라!!!

안 V means "don't/doesn't V"

It is makes a simple statement. It can be used to talk about anyone. It negates the verb.

I like apples. 저는 사과를 좋아해요.

I don't like apples. 저는 사과를 안 좋아해요.

You go to school. 학교에 너 가.

You don't go to school. 학교에 너 안 가.

My boyfriend doesn't go to school. 내 남친 학교에 안 가요.

http://www.talktomeinkorean.com/lessons/level-2-lesson-30/

http://www.learnkoreanlp.com/2009/06/imperatives.html

You have to focus on meaning and not translations when learning a language. They might both use "don't" in their translations, but they have two completely differnet meanings and usages.