As the Lunapads site puts it "women are leaky" :)
Vaginal discharge is the natural process of "flushing out" the vagina. It should be clear and slightly white, with a distinctive but not unpleasant odour. Some women experience more discharge than others, and it increases during certain times of the menstrual cycle and during pregnancy.
Some women find that their daily discharge is enough that their underwear can become damp, so choose to wear pantyliners daily for this reason, others may find they need pantyliners only through some of their cycle. Others may find they do not require pantyliners between periods at all.
If your discharge changes colour, becomes stronger in odour and is accompanied with redness and/or itching, then it could be a sign of an infection, and you should consult a health care professional.