I presume that you are asking about the mass spectrum of string theories.

The mass spectrum of a *Classical* string theory, or the mass of a string is (due to Special Relativity) given, by:

$$m=\sqrt{-p^\mu p_\mu}=\sqrt N $$

In natural units $c_0=\ell_s=\hbar=1$. Where $N$ is an operator, called the "Number Operator". In Classical string theories, this is continuous. When we quantise the theory, we realise that the new mass spectrum is actually given by:

$$m=\sqrt{N-a} $$

Where $a$ is called the normal ordering constant. Now, $N$ is going to take discrete values, multiples of $\frac12$.

In Bosonic String Theory, $a=1$. In superstring theories, $a$ depends on the sector you' are talking about; it is $0$ in the Neveu-Schwarz sector, and $\frac12$ in the Ramond sector.

Of course, In GSO Projected theories (i.e. the tachyon is removed (yes, even in the RNS (Ramond-Neveu-Schwarz) Superstring, there are tachyons if you don't GSO Project; although this problem is absent in the GS (Green-Schwarz) Superstring)) , a GSO Projection gets rid of certain states and so on, but let's keep things simple right now.

Now, I've only been talking about *open strings*. What about the *closed strings*, which are more important, because the open strings are present only in the Type I Superstring theory (and Bosonic, of course (and probably also Type 0A and 0B (not sure))), whereas the closed strings are there in all string theories?

The transition happens to be relatively simple.

You replace $N$ with / $N+\tilde N$ and $a$ with $a+\tilde a$.

**EDIT**

I also see that in your post, you say "strings in particles". Actually, the particles themselves are strings. And they get their mass as per the vibrational modes $\alpha,\tilde\alpha,d,\tilde{d}$of the string with the Number operator $N$ given by

$$ N = \sum\limits_{n = 1}^\infty {{{\hat \alpha }_{ - n}}\cdot{{\hat \alpha }_n}} + \sum\limits_{r/2 = 1}^\infty {{{\hat d}_{ - r}}\cdot{{\hat d}_r}} $$.