Diode
A diode is a device which allows current flow through only one direction. That is the current should always flow from the Anode to cathode. The cathode terminal can be identified by using a grey bar as shown in the picture above.
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. Datasheet here : https://components101.com/asset/sites/default/files/component_datasheet/1N4001%20Diode%20Datasheet.pdf
For IN4148 Diode, the maximum current carrying capacity is 300mA it can withstand peaks up to 2A. The speciality of this diode is its fast recovery time of 8ns at a forward current of 10mA, hence this diode is used in places where fast switching are involved. Datasheet here : https://components101.com/asset/sites/default/files/component_datasheet/1N4148%20Diode%20Datasheet.pdf