With all the different brands that are on the market, figuring out sizing can be tricky. Here are a couple of good rules of thumb to answer your sizing questions.
For Women, jeans are frequently measured by waist size. This is not typically measured at your natural waist, rather below or on your hip bones.
Here is a list of measurements and their conversions to conventional (American) sizes
Inseam: Is measured from the bottom of the crotch to the foot (or on pants from the bottom of the crotch to the end of the pant)
Typical inseams can range from 31"-35" inches. Some brands like Joe's Jeans and Hudson offer longer inseams.
Rises: They are measured from the top of the waist band to the crotch seam.
High Rise- Will fit at or above the navel, typically near the natural waist. (Typically measures 10"-14")
Mid Rise- Will sit right at the navel. (Typically measures 8-9")
Low-Rise- Will sit at least 2" inches below the navel. (Typically measures 8" or below)
The above are meant to serve as guidelines for fit and will of course, vary from brand to brand. It is always a good idea to try jeans on to get a good idea or if ordering online, consult the site's fit guide.
Image from Nordstrom.com