A View is said to be simple view it should have a single 
select statement only.Where as a complex view migh contain 
select statement with joins, sub queries etc...

A View is said to be simple View when it is created 
using a single table.
      A view is said to be comple view when it is created 
on two or more tables

simple view refer the data from only single table. Complex 
view refer data from more than one table. For simple view 
we can use group function(avg,sum etc) but in complex view 
we can use group function.

sorry for previous ans i had some mistakes....

simple view refers data from only single table but complex 
view can refers data from more than one table. In simple 
view we can not use group function(eg.avg,sum...) but in 
complex view we can use group function.

A Simple view selects from one table. A Complex view 
selects from one or more tables. 
A Simple view does not contain functions but Complex views 
contain functions. 
You can perform DML through Simple views but you cannot 
always perform DML through Complex views.