How Cepheid variable stars determine distances to galaxies

[gsonnet911]

Can someone please explain how the distance to stars/galaxies can be determined using Cepheid variable stars? I know that you can find the distance to the cepheid using the period of its contraction/expansion but how is this used?
Im in high school so please use layman's terms :)

 

[mathman]

The period gives the absolute brightness of the star. Observation gives the apparent brightness. Using the inverse square law, the distance can then be calculated.

This approach is based on using nearby Cepheid variables as standards, since their distances can be obtained directly, using parallax.

 

[gsonnet911]

how does this help in finding the distance to non-cepheid stars whose distance away is too great to be measured using the parallax method?

 

[mathman]

The more distant cepheid stars are primarily used to estimate distances to other galaxies. They are of not much help in estimating distances within our galaxy.