Hi Maher,
Flexible is an HMI software.
It is used to program and configure Siemens Panels like OP,TP and MP series.
It can also be used for limited PC applications.
WinCC is a complete SCADA package. It is used to build SCADA projects on PCs and servers and can be used to build server/client architectures.
WinCC is the visualization software of PCS7 (siemens DCS).
There's a massive differences in power tags,scrren objects,message and archive numbers that both can handle.
For example, Flexible 2007 can handle 2048 power tags max. and flexible 2008 (the newest) can handle up to 4000 tags but WinCC can go upto 256K power tags per server and it can handle upto 150K alarm messages.
Both have different licences and for sure WinCC is much more expensive than Flexible.
Configuration and programming,
Flexible is much much easier.
Actually it sounds to be easier but in fact WinCC has much more features and capabilities than what flexible has and that's why flexible looks easier.
You can download brochures for WinCC and WinCC flexible and take a look at their techincal specifications to realize how they are different.
http://www.automation.siemens.com/infocenter/order_form.aspx?tab=3&nodekey=key_517313&lang=en