What exactly is a "fundamental string" in string theory?
For bosonic string theory we have the bosonic string inside a 26-dimensional target space.
Introducing supersymmetry, we have the superstring inside 10-dimensional target space that gives also fermionic string states. In terms of gauge group and SUSY content, superstring theory has several incarnations called Type I, Type II and heterotic.
For this kind of superstring theory, there are so-called D-branes, where D stands for Dirichlet boundary conditions.
Now back to the fundamental strings, what I know is that fundamental strings can attach to D branes.
But in the 11-dimensional M-theory which unifies the different 10-dimensional superstring theories we have M-branes, to which fundamental strings can not be attached.
I have heard of two different kinds of strings, fundamental (or F-strings) and D-strings. What exactly is a fundamental string as opposed to a D-string?