Obsession is having your mind preoccupied with something or someone so much that you cannot stop thinking of it. It can result in one being unhealthily devoted to something. It can also lead to unreasonable or unhealthy ideas and feelings about someone or something.
There is a thin line between passion and obsession. Obsession is a sort of passion. However, it stems from a negative angle. When obsessed, it means you do not have control of your thoughts and feelings. It means that you are less likely to have control over your acts. Examples of obsession could be skipping work or lectures to watch a football match or having erotic thoughts of someone with their pictures all over your wall. For some others, it could be stalking someone they are obsessed with.
Generally, people will always have a passion for different things but it is necessary to be in control of your thoughts and actions in order not to become obsessed.