These diodes are good for reverse polarity protection (put it between your DC power jack and circuitry to avoid a negative-voltage that would fry your electronics), kickback protection (place across your solenoids, relays & DC motors to safely discha
The 1N4148 is a standard silicon switching diode. It's one of the most popular diodes because of its dependable specifications and low cost. Use it for signals up to 200mA of current.