I can confirm what others have suggested: enemy armies abroad in your lands will cause devastation, which cuts into income you would normally receive from farming. Devastation has no effect on taxes or trade income, but if the army positions itself on a road tile such that there is no other unbroken route (roads and ports only), trade will be severely hampered or completely halted (between the settlements linked by the road being blocked, those settlements may still be capable of trade with other neighbours if other routes remain open), just like blockading a port.

I'm not sure, but I believe that the severity of Devastation depends on the size of the army and how many turns it spends lingering.