The Torah tells us not to do "melacha" on Shabbat. Melacha is sometimes defined as "work," but that's not a good definition. What is melacha?
Melacha means "creative act." By refraining from creative acts, we recognize G-d as the Ultimate Creator.
Melacha is any act which represents the uniquely human ability to put our intellect to work and shape the environment. Thus, switching on a light is a melacha. Among other things, it can be considered "building" a circuit.
Specifically, a melacha is anything that fits into one of 39 categories of activities listed in Tractate Shabbat page 73a. This list includes activities such as seeding, uprooting, building, writing and burning.