The Electric-type is a type of Pokémon that have electricity-oriented powers. They possess electro kinetic abilities, being able to control, store, or even produce electricity.
Electric-type Pokémon have varied habitats, from forests, praries, cities and power plants. Electric-type Pokémon are usually fast, and many of their attacks may paralyze the target. Some Electric-type Pokémon also resemble artifical objects used by humans that can relate to electricity (the Magnemite and Voltorb evolution lines and Rotom).
They're weak against the Ground-type because in real life, the ground is a natural sink of electrons.
They're strong against the Flying-type because while in the air, they don't make contact with the ground, so they can't divert the electricity, so they're susceptible to strong electrical currents and it's easy for thunder to hit something that's flying.
They're strong against the Water-type because water is a natural electricity conductor, and if one is in the water, it gets electrocuted as well.
The Dragon-type resists them because in old legends, dragons were immune to the nature forces, electricity included.
The Grass-type resists them because plants and trees are made of materials that don't conduct electricity, like wood and rubber. Also, trees divert the electricity to the ground with their roots.
They resist the Steel-type because all kinds of metal alloys are electricity conductors.