What does the Sentry (Y key) and Trade Network (Control+N keys) keyboard shortcuts do?

I've never used them, and was wondering what their purpose is in the game.

Thanks, and have a merry Christmas to you all.
Happy Civving for 2003. [party]

Sentry: similar to Fortify in that the unit will no longer require orders each turn. It will wake up automatically if an enemy unit approaches.

Trade Network: a form of worker automation in which it will concentrate on linking any disconnected cities to your road network, thus connecting them.

Merry Christmas!