Often pitchers get categorized into what type of starter pitchers are, whether they're an ace, a fifth starter, or third starter at best! It's one way to try classify pitchers, but often it seems to be used lazily (as are terms like fourth outfielders) and often gets biased by what type of team you're looking at (for example, Giants starters are negatively biased due to our strong pitching).
I noticed, and haven't seen mentioned, an article about this on Lookout landing, attempting to define the boundaries of this categories, using tRA+. If you're unfamilar with tRA+ then there's an introduction here.
With respect to the Giants it implies the following categories for our starting rotation:
Lincecum this year has a tRA+ of 152, which is obviously superb. Unsurprisingly that classifies him as a well above average #1 starter. Last year he had a tRA+ of 139 which would still have placed him as an above average #1 starter.
Cain this year has a tRA+ of 111, which is ok, but classifies him as about an average #2 starter. The interesting thing about Cain though, is though that the previous 2 seasons, he's had a tRA+ of 132 & 133, which puts him as a slightly above average #1 starter. Why his tRA+ is so low this year, i'm not sure, but looking at his numbers it looks to be down to the fact that his LD rate is higher this year (20.7 compared to 16.1/16.2) and his GB rate is lower (32.6 compared to 38.6/39.0)
Sanchez this year has a tRA+ of 113, which is decent, & classifies him as about an average #2 starter. Obviously that's a lot better than his ERA suggests, and Sanchez tends to have underperformed his peripherals quite a lot this year.
Zito has had a tRA+ of 92, down slightly from 96, and that puts him as an above average fourth starter.
Correia has had a tRA+ of 83, which is around the #4/#5 starter barrier. Last year his tRA+ was 140 which is pretty impressive, although obviously he was a reliever for a lot of the year, and it's very inconsistant with his previous performances.