Places ramifying in an extension of number fields I came across the following statement in a number theory paper: 
Let $L/K$ denote an arbitrary Galois extension of number fields with Galois
group $G$. Let $S$ be a finite non-empty set of places of L containing all archimedean
places of L (if any) and all those which ramify in the extension $L/K$.
What does it mean for a non-Archimedean place of $L$ to ramify in the extension $L/K$?  
Also can Archimedean places of $K$ ramify in $L$?
Many thanks for your help.

If $\mathfrak P$ is a prime of $L$, and $\mathfrak p$ is the prime of $K$ that 
lies under it, then there is a maximal integer $e$ such that $\mathfrak B^e$ divides $\mathfrak p$. (Alternative, but equivalent, formulation: if we factor $\mathfrak p$ in $\mathcal O_L$, then it is a product of primes $\mathfrak B$ of $L$ to various powers, and the ramified primes are the ones where this power is $> 1$.)
We say that an archimedean prime is ramified if it is of the form $\mathbb C/\mathbb R$ (rather than $\mathbb C/\mathbb C$ or $\mathbb R/\mathbb R$).
