Spaces, or white space, are rendered using some shorthand symbols, or more generally using the \mspace command.
Simple symbols (class 0) are rendered without any space between them. Operators (class 1) are rendered with spaces. Spacing symbols change the amount of spacing, either by adding more space or taking spaces away. Space is measured in math units, or mu. 18mu equals 1em.
...is used for:
a b c d
$ a b c d $
Simple symbols (class 0) have no spaces around them
In some implementations of the mediawiki software, the <math> ... </math> tags try to use HTML in some cases, rather than LaTeX. The standard way to defeat this is to include \, after each mathematical formula.