The pill suppress your body's natural hormonal cycle. This stops ovulation and menstruation and evens out your hormone levels throughout your cycle.
When you get to the inactive pills, you will get withdrawal bleeding because the hormones withdraw from your system. This is not menstruation, but artificially caused bleeding by the pill use. When you start the next pack, the process starts over.
All effects of your normal hormonal cycle may change because the pill evens out your hormones: PMS, achne, cervical mucus, libido etc.
the birth pill contains chemicals called hormones. One type of pill called the combined pill has two hormones called Oestrogen and Progestogen. The combined pill stops the release of an egg every month but doesn't stop periods.