SSH Tunnel/Port Forward

I am trying to grasp the concept of an SSH Tunnel or a Port Forward. 
What I am trying to achieve is a secure communication of a client to 
server. I am also using a router to port forward SSH on port 22 to a 
machine inside a private network, which is running an SSH server 
[MACHINE A]. The client on my machine [MACHINE B] talks to its 
counterpart server MACHINE A on another port, say 7000,.

This is what I have done so far. I have setup, in secureCRT (my SSH 
client of choice) a Local Port Forward whereby, in theory anyway, data 
from MACHINE B on port 7000 should be forwarded via SSH to the router. 
In secureCRT, it also has a setting for a remote port and host name (if 
host is different than SSH server) in which I have added the port 7000, 
thinking that SSH then needs to take the data forwarded from port 7000 
on MACHINE B and send it on to 7000 on MACHINE A.

I can not get this to work unless I also add a port range of 7000 - 7000 
on the router which is forwarded on to MACHINE A. However, I am not sure 
at all if this is going through the SSH tunnel or not.

I know this is a pretty screwed up message but if anyone can help me get 
this going or at least to understand how to do this I would be grateful, 
and will summarize.

Dave
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