Power lines are what carry
electricity from a generating
plant to your
home or
business. They are usually strung on
utility poles but can be
underground as well.
There are many different types of power lines running at many different voltages. You have the long haul, high voltage (250,000 volts and up) lines on massive steel towers, hanging off 30 foot insulators, all the way down to the low voltage 240/120V power lines that only run for a few poles, feeding a few private homes. Then there's the medium voltage lines, running at 14,000V, for example, that run across a district.
Long haul lines run at higher voltages because high voltage reduces loss in the line; for a given amount of current, the line loss is the same, so by increasing the voltage (and thereby reducing the current to deliver the same power, power = voltage * current), you reduce losses as well.
You shouldn't touch power lines... Those who do usually end up well and thoroughly dead. It is also a bad idea to fly kites, model rocketry, and R/C aircraft near them. You probably won't get hurt if you tie two shoes together and throw them up so they hang on the line, though.