char-  should be use when you know that the number of 
characters in the coloumn field is constant .ex if the 
gender is coloumn with char(1)- so only M and F are the two 
options so char(1) is better to use than varchar2.
varchar2- first of all varchar2 is a oracle standard.
varchar2 is nothing but variable characters.
ex- if you have defined the column with varchar2(50) and 
when u r inserting the record into that column with only 10 
letter/characters then only 10 bits will be used instead of 
50.
but if u declare char(50) full 50 bits is allocated for 
that column. so even if u enter on 20 characters the space 
utilized will be full 50 bits.

char and varchar2 both we will use for character purpouse
only.but char take how many bytes we r specified in data
type size it will take that much of size.
eg: x char(10);
x:='nani' the remaining 6 character space will b wasted.
but in varchar2 we can use that 6 bytes for another
character also.
i hope so u under stood very well.