A cepheid is a veriable type of star, wich luminosity variates periodically. The curious thing about this variable stars is that they are the ones that present less variability in ther pulsation periods. They usually change their pulsation period in about 1/50 days .Their fisical estructure is of giant stars, like 10 times the sun.
The most important thing about this stars is that they are used to determinate stelar distances, but how do we calculate distances using the cepheids?
Stellar paralax:
It's an interestellar scale parallax that is used to mesure distances from the earth to other stars using precise astronometry.
The stellar paralax is generally a product of the diferent positions that the earth is in while it's orbiting. That just make the proxim stars aparent to move with respect to more distant stars. If we observe their parallax, mesuring angles and using geometric elements, we can determinate the distance between objects in the space.