The 'backdrop' is a Calabi Yau manifold, which properties I can go into painful algebraic details of if you want. The CY is the space within which the string moves (as well as the usual space-time we see). The string is 1 dimensional, moving in 10 dimensions, 1 time, 3 big spacial dimensions and the 6 in the Calabi Yau. The string is
not part of the Calabi Yau 'edge on' any more than an electron is time 'edge on'.
I'll point out that Ben and I
research this stuff for a living. We've written papers on it (not together). See
these introductory lecture notes where it talks about the world line a particle sweeps out and a world
sheet a string sweeps out. It's a 1d object which moves through time.
The formula I gave for the string's position and motion has 2 coordinates, $$\tau$$ is proper time and $$\sigma$$ is a measure of the position along the length of the string. That's all you need.
And as I explained a few posts up, there's a difference between the fundamental string and branes. The strings define the branes.