The 1N4001 series is a family of popular 1 A (ampere) general-purpose silicon rectifier diodes commonly used in AC adapters for common household appliances. Blocking voltage varies from 50 to 1000 volts.

These are fairly low-speed rectifier diodes, being inefficient for square waves of more than 15 kHz. These devices are widely used for low frequency applications.

For 1N4001 Diode, the maximum current carrying capacity is 1A it withstand peaks up to 30A. Hence we can use this in circuits that are designed for less than 1A.  The reverse current is 5uA which is negligible. It can withstand reverse voltage peak up to 50V.

The cathode terminal can be identified by using a grey bar as shown in the picture above.