What kind of expressions can I use in the Expression Editor?

Hi again, I've used the calculated field feature and find it very useful but what is actually the limitation for the Expression Editor? :) I see from the Cheat Sheet that for text we apparently can use < > operators: "A blank text, good for the <> operator (must be different from blank)" But how? An example is that I would like to check the content in a field and if it is "XXX", then I want to set my calculated field to "YYY". Is that possible or am I stretching it too far? :-) Something similar to this: if Header."Ship-to Code" <> 'XXX then 'YYY'