The following VPNs are for connecting disparate sites to your LAN. See Remote access for information about remote user access. There are several was to establish VPN connections between FortiGates, and some that can be applied to other VPN appliances.
OCVPN is a cloud-based solution to simplify IPsec VPN setup. It automatically generates the IPsec configuration, including static routes and policies, on all of the FortiGates in the FortiCare account. It includes self-learning for updates on a FortiGate, such as changing the public IP address in DHCP.
ADVPN is used in hub and spoke topologies. The hub tells two spokes how they can establish a tunnel between each other, instead of routing traffic through the hub.
Site to site VPNs are used for a single, secure connection between two sites, or between a site and a cloud service. The connection can be to an external party, such as a contractor or MSSP, or within the same business, such as to connect a remote site to the headquarters.