There are various technologies involved in establishing WAN (wide area networks) links. It’s a huge topic. Simply put, there are 3 main technologies-
1. Leased Lines - They are dedicated point-to-point connection between two remote locations. Usually the line is 'leased' from a service provider such as BSNL. Provide high speed, but is costliest.
2. Circuit Switching- In this, in both ends, the devices are connected to the service providers switching network (such as telephone network) via modems. Think like you are making a 'data call' between two sites. Example- Dial-up connections and ISDN (integrated services digital network). This kind of link is cheaper and is used as backup for leased lines.
3. Packet Switching- This is the hottest thing in the market. Here data is sent as small 'packets'. So it's faster, but slower than leased lines. Also cheaper, as same network is shared between other users. One example of this is MPLS (multi protocol label switching) which is now being widely used by corporate.
Now talking about protocols (the logical part of WAN), there are various protocols for WAN such as PPP (point to point protocol), HDLC (high level data link control), ATM (asynchronous transfer mode) etc.