Expert is the full-size board: 30 columns by 16 rows, 99 mines. Nothing about the rules changes, but the board is big enough that pattern fluency and chording speed decide your time more than any single deduction does. Mine density is noticeably higher here than on smaller boards, so you'll spend most of a run flagging and chording through dense clusters rather than reading one number at a time. The endgame is its own skill — once most of the board is open, counting remaining mines against remaining hidden cells often solves the last few cells outright. New to the rules? See how minesweeper works.
Too much? Intermediate is a step down.