An actress who has had major roles in several movies, but is not yet famous enough to be considered a star. Starlets are usually thought of as up-and-coming stars, and it's pretty easy to distinguish the two: If most people know her name and her work, she's a star; if they recognize her but just can't think of her name or where they saw her, she's a starlet. It's impossible to give examples, because starlets become stars at a regular pace (stars don't revert back to starlethood, though; they just slip into the present perfect tense and become has-beens).
In pornography, the standard is a little different. Generally, any woman who appears in a film is called a starlet (of course, there aren't any minor roles in porn flicks, which may account for the difference). Pornporn starstars, on the other hand, are generally people with well-established careers who can sell movies simply by having their name on the box. It's also easy to distinguish the two: If people who aren't into porn are willing to admit that they know the actress's name, she's a star. Otherwise, she's a starlet.