What is Rx/Tx flow control in a NIC, and questions on duplex

[Restek]

I've just purchased some NICs & they contain a setting to enable/disable Rx & Tx flow control. What is this flow control, and what are the benefits/disadvantages of setting it?

While I'm on the subject, I have a linux router/gateway (using the smoothwall distro). I have to set the NIC for the internal network to half duplex - am I alright to set the NIC for the internet to full duplex, or do all nics in the same box need to be set to the same duplex?

[duff]

[Restek]