What is the difference between %%[ ]%% and @variable notation in AMPscript?

%%[ ]%% delimits a multi-line AMPscript code block where you can write SET, VAR, IF, and function calls. @variable is the variable reference syntax — variables are declared with VAR @name and assigned with SET @name = value. Outside a block you reference a variable inline as %%=@name=%%. You can also use inline expressions with %%=FunctionName(args)=%% without opening a full block. The key rule is that executable statements (SET, IF, FOR) must live inside %%[ ]%%, while output references can appear anywhere in the content using the %%=...=%% inline syntax.