You can think of a Ruleset as a table template, but it's more than that. A ruleset is a set (i.e. a group) of rules. When you insert a new table via the Tabulizer editor plugin you are asked to specify the applicable ruleset. All rules that belong to the selected ruleset will be applied during the conversion of the input text to the final table HTML code. In other words, a ruleset is nothing more than a group of rules that all together achieve the desired visual appearance and data transformation.
You can create your own rulesets using the tools provided with Tabulizer, or using a plain text editor. But, if you don't want to go through the learning process or you are not familiar with CSS styling (not everybody is a programmer or web designer after all!) you have the following options: