Microwave ovens use electromagnetic waves of 2.4 GHz to heat food. Unfortunately, the older Wi-Fi standards, 802.11 b/g/n also use electromagnetic waves at 2.4 GHz, but with far weaker transmit power. Although all microwave ovens are well covered, there will always be some degree of "leakage" of waves – a slight leak is not dangerous to humans, but can cause the 2.4 GHz band in the area to become unusable.

Source: Eye Networks