Why port forwarding on port 80?

I have a foscam home security camera and it needs port forwarding on port 80 if I want to use my iphone to view it from outside. Surely enough, it works as soon as I open that port. But isn't port 80 open by default? Else how could I view http websites in the first place. I don't get it. It's already open so why open it again?

Very simply network connections (well, the ones that you're using) are directional. So from your PC on the "inside" of you network, all ports including 80 are (almost certainly) open to everywhere on the "outside". But this doesn't match port 80 *from* the "outside" to the "inside", so these connections are dropped.

There's more to it that this, but it's a good starting point. (Read up on TCP-based network connections and NAT if you want an "interesting" read.)

All ports are open "outgoing", from your PC to the Internet. By default, all ports are closed "incoming" from the Internet through the router (router by default blocks all incoming ports unless a PC application has it open already)

So, port 80 incoming is never open unless you open it.