Vim's scripting feature allows writing custom functions and commands using Vimscript. Vimscript basics: variables assigned with let command, strings use double or single quotes, comments use double quotes, conditional statements use if-else-endif, loops use for-endfor or while-endwhile. Defining functions: function! FunctionName() ... endfunction, using ! to overwrite existing functions. Defining commands: command! CommandName :call FunctionName(). Built-in functions: len() to get length, split() to split string, join() to join strings, substitute() to replace text. Debugging: :echo to output information, :echom to output to message history. Vimscript can be used to write plugins, custom configurations, automation tasks, etc., and is an important tool for extending Vim functionality.