Tags vs. Formulas - How do they differ?
In simple terms, "Tags" are custom groupings of dimension members which may differ from the hierarchy defined within the top-level dimension (for example, defining certain Accounts as “Fixed” vs “Variable” Costs). Tags will not appear on your Dimension page and cannot be used in formulas, but can be fetched like a dimension.
For a more detailed example of how to create tags, see this article.
On the other side, "Formulas" are associated with a given dimension whose value is calculated based on other Cube dimensions. For instance, Gross Profit can be added as a formula in Cube by subtracting "Cost of Goods Sold" from "Revenue". Formula dimensions such as "Gross Profit", can be used in other formulas.
Essentially, the formulas page shows all dimensions that contain formulas, saving you time from having to scroll through the Dimensions page and click on each dimension to see the underlying formula.