Sick describes someone who's not well, suffering from some kind of illness. You shouldn't go to school when you're sick.
If you're sick in bed, you're ill. While sick usually refers to people with physical or mental illnesses, it can also be used to describe an object or system that isn't working right, like a sick economy. Sick can also mean that you're tired of something, like a song you have heard too many times — you're sick of it. Sick is also slang used to describe something that is visually impressive, like a sick hockey goal.