What is a Calorie and what does it have to do with gaining fat?

You always hear health people saying to watch your calories or look how many calories this has and stuff like that. Also stuff like when exercising you have to burn so and so calories. What is a calorie? Is it a sugar or a fat? does it turn into fat? are calories bad for you? aren't calories what gives you strength and energy? what do you know about calories?
if your body is a car, aren't calories the fuel? why would calories be bad for you? why would it make you gain weight?
are calories good for you?