Your engine is built to run, but it runs much better and tends to last longer when it's run at a continuous speed. Imagine running in a race where you constantly went from a full sprint to a dead stop every few minutes. You'd wear yourself out by picking up speed only to force yourself to slow down to a standstill every few minutes. Your engine gets worn down in the same way. Constant stops and starts, like those that you can experience in city driving, are really hard on your car's engine.
If it all possible, stick to the highway and keep your speed and RPMs at a consistent level. You'll notice better gas mileage and your engine will likely last longer if your driving habits allow it to run at a fairly consistent rate. The more stops and starts you throw into the mix the more your engine has to work.