Question : Problem: understanding site to site vpn's

I need help understanding site to site vpn's. When a site to site vpn is established is the connection always active or does the user have to execute a program to connect to the other site. And if the user connects to the other site, how does the connection not get confused  with their present network. Help me understand, I am new to this.  :0)  All feedback is appreciated.

Answer : Problem: understanding site to site vpn's

Typically, a VPN link will remain active while traffic destined for either side's target network is being transmitted, & may remain active for a period of time after VPN traffic is idle, until a "lifetime" timeout is reached.

Your VPN devices are configured to so that any traffic destined for a certain range of IPs will be sent over the VPN tunnel, instead of being sent out as normal unencrypted traffic.

Example:

LAN A <--> VPN firewall A <---> Internet <---> VPN firewall B <--> LAN B
                 |------------ VPN tunnel ------------|

LAN A: IP range = 10.3.2.0/255.255.255.0
LAN B: IP range = 172.18.0.0/255.255.0.0

VPN firwall A is configured so that any traffic sent to the 172.18.0.0 range of IPs is sent via the encrypted tunnel through the Internet (or other connection), any other traffic that's not local to LAN A is sent unencrypted out to the Internet.
   VPN firewall B is configured "in reverse", so that any traffic sent from a LAN b workstation to a LAN A address is sent via the VPN, & if the VPN tunnel isn't active, the VPN connection is established because the VPN firewall sees traffic that it knows must be sent via a VPN link.

Hope this clarifies somewhat.