Reflection of radio waves is significant and can cause reception problems known as "multipath", although it can also enable reception when out of "line of sight" as I mentioned earlier. Most materials will reflect fairly well but large, flat metallic objects are the best (the ground is very good too!). Trees are not very good at all and will absorb most of the signal.
When a RF signal is reflected its polarity is inverted, it is also attenuated by at least 3dB ie half. The amount of attenuation depends on the reflecting material's composition and surface. When the direct and reflected signals arrive at the receiver they may be in phase and add, be out of phase and subtract or be somewhere inbetween. If the receiver is mobile, this may cause multipath fading if the direct signal is weak. This is usually only a problem with AM receivers as FM receivers will "capture" the strongest signal. Multipath reception is the cause of "ghosting" on television receivers as the vision signal is AM.
A great example of multipath interfence has been caused by the building of the towers at Canary Wharf in London. The glass of the buldings is metallised to make it look a pretty silver colour. However the buildings acts as perfect reflectors for the television broadcast signals from Crystal Palalce making televsion reception for some people in the South East impossible.
An example....
Take a PAL televison that has a "ghost" image one fifth of the screen width along from the main image.
There are 625 lines at a frame rate of 25 per second. That gives a line rate of 15.625kHz. That means one line is transmitted in 64uS hence a fifth of a line in 1.28uS. So the "ghost" signal arrives 1.28uS later than the main signal. RF travels at 300,000,000m/S so the ghost signal has travelled 3.84km further than the main signal.