There are a wide variety of semiconductor diodes like  

PN Junction Diode ((IN4007 -  (Small one), DR25 - (Large one) )

DR 25 Clearly exhibits reverse charcateristics.

Zener Diode (pink color one) shown in Left top pic.

In the right top pic older vaccum tube diode is shown. 

In left bottom pic diode symbol with terminals a(anode), k(cathode) is shown. 

In the last figure current direction along with polarities is shown . 

Please observe ALL.

Sri