What exactly is a fundamental string as opposed to a D-string?

Question

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?

Answer 1

A fundamental string (F-string) is the basic building block when we are talking about string theory: a one-dimensional object that may vibrate, and whose states of excitation correspond to particles. These strings may split and merge, just as particles may decay and annihilate to form a new kind of particle. As there are both open and closed strings, it turns out that the open strings can be thought of as ending on a $p$-dimensional hypersurfaces ($p$ being the number of spatial components). These nonperturbative objects, carrying both charge and energy, are referred to as $Dp$-branes. In the case of $p=1$, the D-brane is also referred to as a D-string. A D-string is similar to an F-string, but has a different tension. The role of D- and F-strings is exchanged under S-duality, an $SL(2,Z)$ transformation interchanging weak and strong coupling.