A birth chart shows the locations of the planets at a person's birth and these can be interpreted to provide an insight into an individual's personality. Two birth charts can be compared to evaluate compatibility in a relationship and this is where two people can have planets in different locations.

Example: http://asabovesobelow.com/creativeastrology/Saturn.htm

"If your Saturn is on my Venus trust can allow me to open to you. It doesn't come easily, but when it does you can provide me with clarity and order to achieve love in my life. Without it there is likely to be only fear."

Also see: http://akronastrology.com/_wsn/page2.html
"His Saturn is on my Sun" and

http://www.goravani.com/lessons/Compatibility2.html
"My spouse and I have very high Moon compatibility, but my lagna is 12th to hers, since I'm Aries rising and she's Taurus rising. This "Dwirdwadashamsha" lagna (that means 2/12 relationship) is NOT good for compatibility overall. Furthermore, her Saturn is on my Moon, and other things...."