Transitive group action and meaning of "The usual action of $S_n$"

Question

I ran into an example of a transitive group action. It stated: For $n$ $\ge$ $1$, the usual action of $S_n$ on $\{1, 2, . . . , n\}$ is transitive since there is a permutation sending $1$ to any number. $$\\$$ My questions are basic - firstly, what does "the usual action of $S_n$ on $\{1, 2, . . . , n\}$" mean? Secondly, I know the definition of transitive group action but I'm pretty sure I do not totally understand it. I would like to understand why the above example is an example of a transitive group action (rigorous explanation of what ties the definition of transitive action with this action). $$\\$$ Thank you.

Answer 1

Isomorphic groups can act differently on different sets. For instance the dihedral group of order 8 $D_8$ can act on eight points (a flat square box) giving as generators the elements $(1,2,3,4)(5,6,7,8)$ (a 90* rotation) and $(1,5)(2,6)(3,7)(4,8)$ (flippping the box inside out). It also has an action on a square with rotation $(1,2,3,4)$ and a front-back swap $(1,2)(3,4)$. The number of points do not even have to differ. The Klein Vierergruppe ($C_2 \times C_2$) has an action on four points with even permutations : $\{(), (1,2)(3,4), (1,3)(2,4), (1,4)(2,3)\}$ and another one containing odd permutations : $\{ (), (1,2), (3,4), (1,2)(3,4)\}$. While some groups can have transitive actions they also can have non transitive ones. The dihedral action on the square is transitive but its action on pairs of points is not, adjacent pairs are never mapped to opposite ones.