Technically, lightning is the movement of electrical charges and doesn't have a temperature; however, resistance to the movement of these electrical charges causes the materials that the lightning is passing through to heat up.
If an object is a good conductor of electricity, it won't heat up as much as a poor conductor. Air is a very poor conductor of electricity and gets extremely hot when lightning passes through it. In fact, lightning can heat the air it passes through to 50,000 degrees Fahrenheit (5 times hotter than the surface of the sun).
When lightning strikes a tree, the heat vaporizes any water in its path possibly causing the tree to explode or a strip of bark to be blown off.
