The 1N4148 diode. The black band at one end of the 1N4148 diode indicates the cathode (negative side). Current flows from the anode (positive side) to the cathode, but not the other way. (Some types of diodes have a silver band instead of a black band to indicate the cathode.)
The 1N4148 is the neat looking orange-and-black diode, where the black line marks the negative (cathode) side. The 1N4001 is the black and gray diode, and the gray line marks its cathode.