I will do my best to summarize these questions, but you may wish to look at a physics book for more detailed answers.
Current is related to the flow of electrons through something. The more electrons are flowing, the bigger the current is. The only catch here is that current flows in the opposite direction of electrons. For example, if electrons are moving right, current is moving left. This is because the direction of a current in a circuit is defined as the direction positive charges would move, and electrons are negative.
Voltage has to do with potential energy. The important thing to know about voltage is that it is only meaningful when you're looking at more than one point. For example, if a bird lands on a high voltage power line, it doesn't matter because it's not connected to anything with a lower voltage. If, however, you stand on the ground and touch a high voltage (compared to ground) power line, you will have current flowing through you since current flows from places with higher voltages to places with lower voltages.
A resistor is an example of a load. Resistors restrict current flow through a circuit. (The electrons don't easily get from one side to the other.)
Now onto your other question....
High voltages are used in power lines because you are trying to send power from one place to another. Power is roughly equal to the current times the voltage. That means that if you want to send a lot of power you can use:
a) high current
b) high voltage
High voltage is actually needed for to send power over long distances because if you use high current, a lot of the power is lost to the resistance in the wires. This doesn't happen as much with high voltage since the currents are smaller!
(republished on 07/22/06)
(published on 03/03/07)
Since power is voltage times current, high voltage lets you use lower current. That means you can use smaller wires.
(published on 08/16/13)