Just install Ubuntu 13.10 on my second PC to try a Linux OS. I'm working as developer on windows based platforms but I'm completely fresh in Linux/Ubuntu.
When I try to connect to WiFi (WiFi adapter seems to work properly- I can see the networks), I enter the password (I'm using WPA2-PSK) hit 'connect' but after couple of seconds I receive the authentication dialog again (I double checked the password- It's correct). It's seems that the network adapter can't authenticate the connection with the router although the password is right.
My Network adapter: Realtek RTL8187SE Wireless Lan Controller.
As offered in several others answers, I tried to remove and forget the network that I trying to connect, restart the system and connect again. But it didn't help.