I am currently reading these lecture notes about string theory: http://www.phys.uu.nl/~thooft/lectures/string.html I think that I understand the general idea. After that it's simply applying QFT formalism, which I'm more or less familiar with (or so I thought). But in Chapter 7. - BRST quantization of the notes (p. 31) I encountered a few concepts and terms that I don't what mean: What's the D in equation (7.1) (does it have anything to do with the covariant derivative?) and what are "gauge orbits"? I haven't found anything about BRST quantization in Landau and Lif****z's Quantum Electrodynamics, which is the only book I have on the subject. Google only gave me links to scientific papers exploring the procedure. Also on page 34, they associate the world sheet of two interacting closed strings with the topology of a sphere, and the strings being topological "holes" in the sheet. It was my impression that the world sheet of a string was the string itself (p. 8); the two dimensional world sheet (proper time and a spatial coordinate along the string) is located in a higher dimensional space-time. I thank you in advance for your help.