I would say that pretty much any girl who is in the porn industry is considered a 'porn star'. There are clearly different degrees though.
You have your girls who are popular on the internet and reasonably well known amongst fans and followers of porn. Someone like Gianna or something.
Then there's the girls who are really popular in porn but still not as recognisable as some. Girls like Eve Lawrence who have a lot of titles out and have a strong following.
Then there's the girls who are in the massive porn productions and are contracted to the big companies like Vivid with a lot of fans on the 'net and are recognisable to casual porn watchers. Girls like Jesse Jane, Krystal Steal, Devon etc.
Then above even that, there's the girls who have broken the mainstream and are recognizable as celebrities. The obvious one being Jenna Jameson but lately Tera Patrick has become pretty mainstream.