style system is how you customize the output produced by your view. A view style is basically a smart theme template that processes the view data and then outputs it. All styles in Views can be overridden by placing copies of the templates in your theme directory and then modifying them. See the theme: information link available on all views to get hints for which templates a given view is using.
A breakdown of View output
By default, the style is unformatted, which means that there is very little style actually used; the records are simply displayed one after another, enclosed in a <div> tag so that you can use CSS to manipulate the view.
Some styles use a separate row style to determine how each row of the View looks. This is useful for mixing and matching styles to more readily produce exactly the kind of output you need.
Many styles can be grouped. For styles that can, there will be a 'grouping field' option; pick one of the fields to group by. Please see Grouping in styles for more information.
Each style is its own entity.
If you want your fields to be templatable, you need to use values that are acceptable in function names. That's actually more strict, since that basically limits you to alphanumeric and _. Even though Views may be able to query datastores that use special characters, naming is still important for use in templates.