Your fans are always on when your Mac is on, spinning at a minimum of 1800 rpm. They will spin faster as needed to keep temps at a safe level. If they're spinning up without increased heat, try
resetting the SMC.
iStat Pro will give you accurate readings of your temps, among other things. Also, make sure you don't block the vents, which are located at the rear, near the hinge.
Websites with Flash content, games and other multimedia apps will put higher demand on the CPU/GPU, generating more heat. This is normal. If you're constantly putting high demands on your system, such as gaming or other multimedia tasks, expect temps to rise and fans to spin up accordingly. It's just your Mac doing its job to maintain temps within the normal range.