Outside of the other help articles covering the calculation field, You can find more details on the calculation field on our developer documents page. Once on the developer documents page, select "Calculation Expressions" to reveal a drop-down pick list.
On the introduction page, you will find a short overview of calculation fields.
On the Reference page, you will find details on the various custom functions that you can use in calculation fields.
On the Examples page, you will find example code that can be used within calculation fields.