quantum mechanics - Expectation values in QFT?

What is the meaning of different expectation values in QFT?

For instance:

$$\langle 0|{\cal O}(0)|q,s\rangle$$

or

$$\langle 0|{\cal O}(0)|0\rangle$$

with ${\cal O}$ being some operator and $|q,s\rangle$ some single particle state with the written quantum numbers.

For instance, in Srednicki's textbook I mostly see expectation values of the second kind, i.e. $\langle 0|{\cal O}(something)|0\rangle$. How come?