The terms Fixed Costs and Variable Costs look at the behavior of the costs - do the costs vary with Sales Volume?
Variable Costs are costs that vary with the Sales Volume. If Sales double from one month to the next, then the Cost of Sales will also double. Cost of Sales is a Variable Cost.
Fixed Costs are costs that do not vary directly with Sales Volume; they are approximately the same from month to month (they are ‘fixed’). Examples include R&D, advertising, non-production personnel, training, and rent for the corporate offices. If you double advertising from one month to the next, you will not double Sales.
The terms Cost of Sales, Direct Costs, and Variable Costs are very similar. Check how each term is used in your company.
See also Fixed Costs and Semi-Variable Costs.
term list.