In math mode, 
ignores all spaces. It has complicated
built-in rules which it uses to determine how the equation
should be spaced. (For instance, it knows to put a little extra
space around + and = signs.)
But sometimes you want to put words in the middle of an
equation. For instance, you might want to generate
The way you do this is to use the \text{ } command to
tell 
to go back into text mode briefly. In our example,
you would type
$$ \{\psi \mid \psi \text{ is an additive character}\} $$.
Then will set ``is an additive character'' with a space
before it in text mode. Note that if we had left out the space
before ``is'', we would have got
This brings up a point about using math mode properly. In math
mode, assumes that any string of letters next to one
another is a product, and spaces it accordingly. If, in fact,
it was a word, it won't look good. So, for symbols which are
actually parts of words, like Stab, Spec, Proj, etc., you should
use \operatorname{ } to tell to space the letters in the
symbol as text, not as a product. For instance, type
$$ X := \operatorname{Spec}A $$
to get
You can make subscripts and superscripts of symbols you have generated
using the \operatorname{ } command: e.g.,
$\operatorname{Stab}_G(H)$
gives 
.