Wireless routers broadcast like any other radio transmitter. The strongest signal is broadcast from the centre of the antenna and weaker near the top and bottom tips. Radio waves vary in frequency - lower frequency waves travel through walls while higher frequencies will not. The power output on these things is very, very low, somewhere around 100mW. To compare, microwaves vary between 650W-800W (although they are shielded). Radio waves are all around you. Mobile phones/wireless phones/commercial radio transmitters/TV/satellites/TV senders/handheld walkies/CCTV.
As for harmful effects - there is no scientific proof that there is any harm living among all these radio fields. Wireless routers are right on the bottom end of the scale as far as power output is concerned. I'm not sure I'll see any proof one way or another in my lifetime - so, I'll enjoy the benefits for now!