The most annoying thing one can do when typing. Very difficult to read as well. When one sees someone typing like that, they should be FLAMED WITHOUT MERCY.

there are many ways to write each letter in leet.
here are the most common:

A= 4
B= 8
C= (
D= |)
E= 3
F= |=
G= 6
H= |-|
I= !
J= _/
K= |<
L= 1
M= /\/\
N= /\/
O= 0
P= |>
Q= &
R= |2
S= 5
T= 7
U= |_|
V= \/
W= \/\/
X= ><
Y= '/
Z= 2