When dealing with car batteries, a red lead, terminal or cover marks the positive end of the battery. The black lead, or plain metal in some cases, is the negative end.
Understanding which terminal is positive and which is negative is vital for safety when dealing with a car battery. When replacing a car battery, it is important to disconnect the cable going to the negative terminal first and then the positive terminal. When connecting jumper cables to a car battery, linking the positive terminal from the donor battery to the negative terminal on the recipient battery, or vice versa, leads to an electrical hazard.