LookoutDirect as SCADA

Could anyone tell me what is a difference between OPC client and server?
I am going to use LookoutDirect as a SCADA and the customer wants to be able to dial into it from remote location and make the machine Start and Stop.
In manuals it is said that LookoutDirect has only OPC client capabilities. Is it enough for my application?

Lookout HMI

Not sure if this will be any help but I have been playing with a demo disk from National Instruments for thier Lookout HMI (this is the company that AutomationDirect buys LookoutDirect from). They provide a package that includes 1 server and 4 clients with unlimited I/O and free drivers for most major plc's. AutomationDirect just sells a limited version with a max 500 I/O's and named it LookoutDirect. AutomationDirect provides a better driver that they developed for thier own plc's.

I was curious what plc you are using (05,06,240,250....)?

I understand that the server provides programming development capability and the clients access data from the server to thier process control or something like this. Hope someone else will shed better light on this but I believe that the client can control points also (turn motors on and off)

In both Lookout (version 4.5 or 5.0) and LookoutDirect (version 4.5) the software is an OPC server. Data is available to most any OPC client. This should not be confused with Lookout client and server licences. However, regardless of the above info, you can easily control your application from a remote location.

From my limited experience with OPC the server application interfaces with the PLC. The Client application interfaces with your database or HMI applications and the such. The Server and Client interface with each other.

In Generally:
client ask something from server and server send answer to client?
and separately:
F.E. InTouch (client) ask some i/o from OPC-Server.
OPC-Server (now it is client for PLC) ask it from PLC.
PLC (Server) sends asked i/o contents to OPC-Server and
OPC-Server forwarding i/o contents InTouch. Is it so ?