Abstract syntax trees (ASTs) represent the recursive structure of a formal document (program source code).
Abstract Syntax Trees (abbreviated "AST") represent the structure of a formal document (often a computer source program). Nodes in the tree represent syntactically significant chunks of document (function definitions, declarations, statements, expressions and subexpressions). Children of a node represent the parts of that chunk (for a function definition node, the children are likely to be "name", "signature" and "body").
ASTs are widely used in program analysis and transformation systems, as well as classical tools such as compilers.
They are typically constructed by a parsing activity, which is driven by the BNF rules that describe the formal document structure. If one captures precisely how the parser matches the BNF rules to the formal document, the result is a so-called concrete syntax tree (CST). Usually CSTs contain a lot of unnecessary detail (such as parentheses and keywords) that are not needed by the tool using the tree, since the tree nodes themselves essentially represent grammar rules and this information is thus redundant. So parsers often construct ASTs during the parsing process to produce relatively compact trees compared to what a CST would be. See What is the difference between an Abstract Syntax Tree and a Concrete Syntax Tree? for more discussion on this topic.
There is an "ast" tag for StackOverflow; you should use the "abstract-syntax-tree" tag instead.
