Lunar and solar eclipses do not happen every month because the orbit of the moon and Earth are not on the same orbital plane. The moon's orbit has an incline of approximately 5 degrees in relation to the Earth's. This means that the moon, sun and Earth will not line up as often to create the conditions that are needed to create a lunar or solar eclipse.
Lunar and solar eclipses occur when all three celestial bodies line up in a specific way. A lunar eclipse occurs when the moon is in its full moon phase and passes through the Earth's shadow. These eclipses can be partial or can be full depending on the alignment of the moon, Earth and where a person is viewing the eclipse. Not all eclipses are able to be viewed from everywhere on Earth, especially not at the same angle due to the shape and tilt of the Earth at the time of the eclipse. A similar alignment occurs with a solar eclipse. The moon in this case is in its new moon phase, and lines up with the sun in such a way that people in certain areas of the Earth will view a full solar eclipse. The distances of the moon and sun from the Earth keep them appearing the same size to the human eye, so when the moon moves in front of it, it blocks the view of the sun.