XQuery is an interesting language with some unusual ideas. This article provides a high level view of XQuery, introducing the main ideas you should understand before you go deeper or actually try to use it.
The first thing to note is that in XQuery everything is an expression which evaluates to a value. An XQuery program or script is a just an expression, together with some optional function and other definitions. So 3+4 is a complete, valid XQuery program which evaluates to the integer 7.
There are no side-effects or updates in the XQuery standard, though they will probably be added at a future date. The standard specifies the result value of an expression or program, but it does not specify how it is to be evaluated. An implementation has considerable freedom in how it evaluates an XQuery program, and what optimizations it does.