Understanding mathematical set theory syntax While reading a paper, I came across some operators and syntax that I am not familiar with, and I'm not sure how to google to find the definitions or explanations I am looking for. 
For reference, I am reading this paper, page 6.
In this line:
    $$f\colon S \to 2^{R \cup AR}$$
I understand that this means- $f$ is a function that maps from $S$ to the union of the set of all $R$ and $AR$. But what is the significance of mapping $S$ to $2$ raised to this union?
On the same page, there are a couple expressions involving blocks in $[]$ tags enclosed in $\{ \}$. I know that $\{\,r \mid\dots\}$ reads, the set of all $r$ such that .., but what is the significance of $\{\,r \mid\dots [\dots]\}$. How does this read in English? Examples are in the same page on the linked paper, my formatting skills aren't good enough to type it out here. 

Well, the set
$$Y^{X} = \{ f\colon X\to Y\}$$
describes the set of all functions from $X$ to $Y$. 
So $2^{X}$ are the indicator functions on all possible subsets of $X$. This is usually identified as the powerset of $X$, i.e., the set of all possible subsets of $X$.
So
$$f\colon S\to 2^{X}$$
describes a function $f$ from a set $S$ to the powerset of $X$, i.e., assigns to each element $s\in S$ a subset $f(s)\subset X$. NB: it is entirely possible that $f(s)=\emptyset$.
Example. Consider the set $A=\{1,2\}$. Then we have 4 elements of $2^{A}$, namely 
$$\chi_{1}(x)=\begin{cases}1 & x=1\\
0 &\mbox{otherwise}\end{cases}$$
and likewise $\chi_{2}(x)=1$ if $x=2$, and $0$ otherwise. Then we have $\chi_{0}(x)=0$, which is the characteristic function for the empty set...and $\chi_{1,2}(x)=1$ for any $x\in A$.
Observe these are the characteristic functions for $\{1\}$, $\{2\}$, $\emptyset$, and $A$ (respectively).
Point 2. Regarding the notation of square brackets [] inside the set, this is an obscure notation used by logicians sometimes. They use quantifiers $\exists$ or $\forall$ and then some predicate afterwards, and the predicate is in square brackets. So, for example:
$$(\exists y)[y<2\land y>0]$$
This is a proposition "There is some $y$ such that $y<2$ and $y>0$."
The quantifier portion is placed in parentheses, the predicate inside square brackets.
