To give you some personal background: I am a ring theorist, and most of my research focus on invariant theory of noncommutative rings. Recently I became interested in a certain problem that requires a rather solid understanding of the birational classification of projective curves.
The most important thing about the introduction to the birational classification of curves I'm looking for is being short and/or concise, quick to absorb, and include a discussion of the Hurwitz-Riemann formula. If it is possible, I would prefer a source that: 1 - does not use unecessary heavy machinery from algebraic geometry and 2 - works over any algebraically closed field. About 2-, in fact, I'm okay with just algebraically closed fields of zero characteristic, since this is the case I need (but I don't know if restriction of the characteristic makes any signifficant difference in the classification).
I stress that 1- and 2- are optional. For instance, if the quickiest way to learn the subject is over $\mathbb{C}$ using some theory of Riemann surfaces, I am ok with that.