What is a "shared" WiFi key?

I'm trying to use NetworkManager on a WiFi network
with a WEP encryption key.
When I try to like to the network
I am asked (reasonably) for the key.

But I am also asked if it is a "shared" or "open"
network or key.

What exactly is the difference between these?

[As it happens, NetworkManager does not accept the key
when I type it in, whether I opt for "shared" or "open".
But that is probably a different question.]

--
Timothy Murphy
e-mail (<80k only): tim /at/ birdsnest.maths.tcd.ie
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College, Dublin 2, Ireland