A general photon isn't too dangerous. Most photons that we encounter have the power to heat our bodies and not much else. The heat we absorb from photons daily isn't that much, so this is rarely a problem.
Now, an interesting thing about photons is that two photons of a lower energy do not make a single photon of higher energy (frequency). So a million visible photons still will not have the same effect as a single ultraviolet photon. For example, if a certain chemical bond requires a UV photon to be broken, shooting a lot of visible photons at it won't work.
Ultraviolet photons have the capabilities of mutating DNA and other essential molecules. Too much of it, and it's likely that you'll get skin cancer. Our bodies are tuned to be able to deal with a small amount of UV radiation (which e experience daily), so it's usually not a problem. If you're planning to be out in the sun, sunblock helps keep you doubly safe.
Gamma photons pass right through skin and affect other molecules inside our bodies. Again, our bodies can deal with a small amount of gamma radiation, but if it's a powerful beam then (aside from overheating), many essential chemicals in our body will be broken down into (possibly toxic) fragments.
The magnetic field isn't that effective to keep out photons1, but the Earth's atmosphere keeps out most UV/gamma radiation. Astronauts in space need special filters in their space suits (and in their shuttles) to avoid being burned by cosmic rays.
1 Charged components of cosmic rays like high-speed muons and various hadrons matter are prevented from entering by both the magnetic field and the atmosphere (the combined effect causes the aurorae). These (and their decay products) have the capability of doing quite a bit of harm if they came unhindered to the surface.