Load Balancing/Sharing on Linux OS

hi,

i have 2 isp services, ADSL cables. then, how do i maximize the usage of both cables. i wish to have something like, if i have one cable failed-down, then, other one will take turn on service. if both lines are ok, then, they will sharing the incoming and outgoing traffic from my LAN to the internet.
so, what is the thing that i should look into? which area????

thanks....