The 'space weather' have been coined to point out that many of the phenomena studied in space physics have also important practical consequences (e.g., Davenport, 1996). In addition to the geomagnetic activity, also changes in the atmosphere, or even comets and man-made debris in space, can be of importance. There is even a real possibility that the much talked about 'global change' may be - at least partly - due to changes in our Sun.
The most obvious effect of geomagnetic activity (usually CME driven storms) is the build up of enhanced ionospheric current system within the equatorward moving auroral oval. As a consequence, current surges can be induced in the power lines, causing flickering lights and black-outs resulting in millions of dollars worth of damage. In addition, telecommunication cables and even petroleum pipelines are affected.
The other serious consequence of bad space weather is the possibility of damage to Earth orbiting satellites. Both storm time medium energy CME particles and flare related high energy solar (cosmic ray) particles can do the trick. The damage can a single instant breakdown of some internal electronic component, or increased deterioration of spacecraft surface materials, sensors, and solar panels. There are quite a few communication satellites up there providing us telephone links, television programs etc. that we are dependent on. Other things that are harmful to satellites include repeated passages through radiation belts, and hitting man-made debris or comets (the radiation belt intensity is related to space weather). Note also that satellite may suffer of 'single event upsets' (SEU) due to energetic particles. Finally, everything that causes these problems to satellites, most likely causes problems to humans in space!
Another way the satellites can be affected is due to heating and expansion of the upper atmosphere, that leads to increased neutral density and drag at satellite orbit. Geomagnetic activity is an important source of enhanced Joule heating, and it is possible that flare related X-ray radiation could do some heating, too.
Solar flares affect the communication in other ways, too. They produce radio frequency interference (solar radio noise) trough their radio bursts, and degrade high frequency radio propagation at high latitudes via polar cap absorption (PCA). In addition, their X-ray bursts cause sudden ionospheric disturbances (SID) that, together with PCA, affect LF and VLF navigation systems.